Sqlite errors (#601)

* Null checks on return from std::make_shared are unnecessary...

... instead std::bad_alloc would be thrown in case of memory exhaustion, which
we don't try to recover from.

* Restore uses of the mayThrow arguments in Sequence...

... that became unused at commit d39590cf41.

It's important to ignore exceptions from SampleBlocks when only displaying, not
editing or playing, and just treat missing data as silence.

Pass the boolean into the SampleBlock routines.  But the throwing of exceptions
is not yet implemented.

* SampleBlockFactory functions guaranteed to return non-null or throw...

... which corrects Sequence.cpp, which was assuming non-null results.

This supplies the throw statements that the previous commit comment says were
still lacking.

This corrects the absence of checks of returns from sql_bind_... function calls
in SqliteSampleBlock.cpp.  (Other calls remain to be checked elsewhere.)

User visible error messages, carried by the exceptions, might be improved.

* Restore the try/catch in AutoSaveFile::Decode...

... which was introduced at 2ba17c78d6

but removed at d39590cf41, yet without removing
the throw statement

which left the program vulnerable to abrupt termination instead of graceful
failure, when uninterpretable auto save contents are detected.

src/SampleBlock.h

@@ -31,9 +31,9 @@ using SampleBlockID = long long;
 class MinMaxRMS
 {
 public:
-   float min;
-   float max;
-   float RMS;
+   float min = 0;
+   float max = 0;
+   float RMS = 0;
 };
 
 class SqliteSampleBlockFactory;
@@ -52,10 +52,12 @@ public:
    
    virtual SampleBlockID GetBlockID() = 0;
 
-   virtual size_t GetSamples(samplePtr dest,
+   // If !mayThrow and there is an error, ignores it and returns zero.
+   // That may be appropriate when only attempting to display samples, not edit.
+   size_t GetSamples(samplePtr dest,
                      sampleFormat destformat,
                      size_t sampleoffset,
-                     size_t numsamples) = 0;
+                     size_t numsamples, bool mayThrow = true);
 
    virtual size_t GetSampleCount() const = 0;
 
@@ -65,14 +67,29 @@ public:
       GetSummary64k(float *dest, size_t frameoffset, size_t numframes) = 0;
 
    /// Gets extreme values for the specified region
-   virtual MinMaxRMS GetMinMaxRMS(size_t start, size_t len) = 0;
+   // If !mayThrow and there is an error, ignores it and returns zeroes.
+   // That may be appropriate when only attempting to display samples, not edit.
+   MinMaxRMS GetMinMaxRMS(
+      size_t start, size_t len, bool mayThrow = true);
 
    /// Gets extreme values for the entire block
-   virtual MinMaxRMS GetMinMaxRMS() const = 0;
+   // If !mayThrow and there is an error, ignores it and returns zeroes.
+   // That may be appropriate when only attempting to display samples, not edit.
+   MinMaxRMS GetMinMaxRMS(bool mayThrow = true) const;
 
    virtual size_t GetSpaceUsage() const = 0;
 
    virtual void SaveXML(XMLWriter &xmlFile) = 0;
+
+protected:
+   virtual size_t DoGetSamples(samplePtr dest,
+                     sampleFormat destformat,
+                     size_t sampleoffset,
+                     size_t numsamples) = 0;
+
+   virtual MinMaxRMS DoGetMinMaxRMS(size_t start, size_t len) = 0;
+
+   virtual MinMaxRMS DoGetMinMaxRMS() const = 0;
 };
 
 ///\brief abstract base class with methods to produce @ref SampleBlock objects
@@ -90,17 +107,44 @@ public:
 
    virtual ~SampleBlockFactory();
 
-   virtual SampleBlockPtr Get(SampleBlockID sbid) = 0;
+   // Returns a non-null pointer or else throws an exception
+   SampleBlockPtr Get(SampleBlockID sbid);
 
-   virtual SampleBlockPtr Create(samplePtr src,
+   // Returns a non-null pointer or else throws an exception
+   SampleBlockPtr Create(samplePtr src,
+      size_t numsamples,
+      sampleFormat srcformat);
+
+   // Returns a non-null pointer or else throws an exception
+   SampleBlockPtr CreateSilent(
+      size_t numsamples,
+      sampleFormat srcformat);
+
+   // Returns a non-null pointer or else throws an exception
+   SampleBlockPtr CreateFromXML(
+      sampleFormat srcformat,
+      const wxChar **attrs);
+
+protected:
+   // The override should throw more informative exceptions on error than the
+   // default InconsistencyException thrown by Create
+   virtual SampleBlockPtr DoGet(SampleBlockID sbid) = 0;
+
+   // The override should throw more informative exceptions on error than the
+   // default InconsistencyException thrown by Create
+   virtual SampleBlockPtr DoCreate(samplePtr src,
       size_t numsamples,
       sampleFormat srcformat) = 0;
 
-   virtual SampleBlockPtr CreateSilent(
+   // The override should throw more informative exceptions on error than the
+   // default InconsistencyException thrown by CreateSilent
+   virtual SampleBlockPtr DoCreateSilent(
       size_t numsamples,
       sampleFormat srcformat) = 0;
 
-   virtual SampleBlockPtr CreateFromXML(
+   // The override should throw more informative exceptions on error than the
+   // default InconsistencyException thrown by CreateFromXML
+   virtual SampleBlockPtr DoCreateFromXML(
       sampleFormat srcformat,
       const wxChar **attrs) = 0;
 };