Help button for many warning messages; Restored warning icon.

The error icon was gone because we are now using the ErrorDialog, which didn't have it.  So added back explicitly.  I decided to go with wxART_WARNING rather than wxART_ERROR because nearly all of these present as warnings.

src/FileException.h

@@ -51,6 +51,7 @@ public:
 protected:
    //! %Format an error message appropriate for the @ref Cause.
    TranslatableString ErrorMessage() const override;
+   wxString ErrorHelpUrl() const override;
 
 public:
    Cause cause;