Review and standardise error messages

[AlisterH]

Currently GWC produces error messages with a range of wording for the same type of error:
"Cannot write to [filepath.toc]" (my preferred wording, only just implemented, for the secret append to cdrdao feature).
"failed to save selection" (save selection as)
"Unable to open [filepath.toc]: permission denied" (both cdrdao menu entries)
"can't init outfile '[filepath.mp3]'", only output to the terminal for both mp3 entries (there is no indication in the gui that the action failed, which is Not Good).
We should probably try to standardise the wording. I suspect all three toc options and save selection will only fail by simple write errors, so my preferred wording is OK. Mp3 and ogg encoding might fail for other reasons - we should probably just figure out how to capture the error produced by the encoder and present it to the user. Or maybe it would be better to just show the user all of the encoder output.