Sqlite open bugs (#603)

* Docs say: call sqlite3_close even when sqlite3_open returns error

* Be careful of possible failures of AutoSave...

... In ProjectHistory operations, do the AutoSave first and throw if there is
a failure.

Only then change UndoManager's state, confident that it remains consistent with
what was AutoSaved.

* Throw exceptions if lazy opening of project's database fails...

... because the calls to DB() as in Sqlite3SampleBlock may be in deeply nested
places that can't propagate the error codes; and besides, those functions
had been assuming non-null returns from DB(), which might have crashed before,
but now the assumption is correct when the throw didn't happen.

Note that this exception may also happen during attempted Autosave.  Uses of
Autosave were reviewed and some changes made in the previous commit.

src/ProjectFileIO.cpp

@@ -235,12 +235,15 @@ ProjectFileIO::~ProjectFileIO()
 
 sqlite3 *ProjectFileIO::DB()
 {
-   if (mDB)
+   if (!mDB)
    {
-      return mDB;
+      OpenDB();
+      if (!mDB)
+         throw SimpleMessageBoxException{
+            XO("Failed to open the project's database") };
    }
 
-   return OpenDB();
+   return mDB;
 }
 
 sqlite3 *ProjectFileIO::OpenDB(FilePath fileName)
@@ -266,6 +269,9 @@ sqlite3 *ProjectFileIO::OpenDB(FilePath fileName)
    if (rc != SQLITE_OK)
    {
       SetDBError(XO("Failed to open project file"));
+      // sqlite3 docs say you should close anyway to avoid leaks
+      sqlite3_close( mDB );
+      mDB = nullptr;
       return nullptr;
    }
 
@@ -647,6 +653,8 @@ bool ProjectFileIO::CopyTo(const FilePath &destpath)
    }
    else
    {
+      // sqlite3 docs say you should close anyway to avoid leaks
+      sqlite3_close( destdb );
       SetDBError(
          XO("Unable to open the destination project file:\n\n%s").Format(destpath)
       );
@@ -1272,6 +1280,8 @@ bool ProjectFileIO::SaveCopy(const FilePath& fileName)
    rc = sqlite3_open(fileName, &db);
    if (rc != SQLITE_OK)
    {
+      // sqlite3 docs say you should close anyway to avoid leaks
+      sqlite3_close( db );
       SetDBError(XO("Failed to open backup file"));
       return false;
    }