Define an AudacityException subclass for file errors

src/FileException.cpp

@@ -0,0 +1,45 @@
+//
+//  FileException.cpp
+//  
+//
+//  Created by Paul Licameli on 11/22/16.
+//
+//
+
+#include "Audacity.h"
+#include "FileException.h"
+
+FileException::~FileException()
+{
+}
+
+std::unique_ptr< AudacityException > FileException::Move()
+{
+   return std::unique_ptr< AudacityException >
+      { safenew FileException{ std::move( *this ) } };
+}
+
+wxString FileException::ErrorMessage() const
+{
+   wxString format;
+   switch (cause) {
+      case Cause::Open:
+         format = _("Audacity failed to open a file at %s.\n");
+         break;
+      case Cause::Read:
+         format = _("Audacity failed to read from a file at %s.\n");
+         break;
+      case Cause::Write:
+         format = _(
+"Audacity failed to write to a file at %s.\nAttempt this operation again after removing unnecessary files.\nOne way to do that is with the Discard buttons in the History dialog.\nSee the View menu.");
+         break;
+      case Cause::Rename:
+         format = _(
+"Audacity successfully wrote the file %s but failed to rename it as %s.");
+      default:
+         break;
+   }
+   return wxString::Format(
+      format, fileName.GetFullPath(), renameTarget.GetFullPath() );
+}
+