If an import fails should partial import be left or removed? issues/144 

This is a rebase and single commit version that succeeds from
https://github.com/metapolator/metapolator/pull/212/

Simply, this PR lets loading of a UFO have failures without stopping the entire operation. If a glyph fails then it is recorded as such and why, but the user can still import the UFO for use with metapolator (obviously without the offending glyphs).