format None values as the empty string

Fix due to @pscn. Includes test. Closes #108.
fix bytes literals causing a Unidecode warning

This would trigger a warning in Unidecode when metadata was missing (which is
the only case when those empty-string literals are used). Closes #109, which
is a different fix for the same problem.
mbcollection: human-readable MB exceptions

Fixes #107 by wrapping the API invocation function with exception handlers.
convert: add missing util.displayable_path