[fontir] Start to refactor errors

[cmyr]

This is some initial cleanup of the errors in fontir; the next step is going to be to unify Error and WorkError.

• Replace various Error variants with a 'BadSource' error that packages up a particular error cause with the path of the problematic source
• Remove the general 'io error' variant from Error in favour of finer-grained errors that include additional context.
• get rid of dyn Error