error codes in convert-ly

[Graham Percival]

It would be nice if convert-ly could report a failed exit code if it 
failed to convert a file.  I spent a few minutes looking at the script, 
but I couldn't figure out how to safely add it myself.

ie "Not smart enough to convert" also produced a sys.exit(1)


This would be useful for our internal testing: when you're updating 50 
lilypond files, you can't look at the shell output to check if 
everything updated correctly.