song-io: add GError parameters to API and a GError enum

Add support for GError so that we can improve our reporting of what exactly did
not work. The errors are not yet set by the code nor are they reported.
See issue #22.
song-io: set GError on all error branches

Set a SongIOError code and message on all error path or fall back to GIOErrors.
docs: add my last name to the custom dictionary
buzztrax-cmd: add GError for song-io.

Print the messages to stderr.
Fixes #21 for buzztrax-cmd.
buzztrax-edit: add GError handling for song-io.

Pass the messages to the error dialog.
Fixes #21 for buzztrax-edit.
design: add dialog mock

Study for issues #21.