Fix handling of io.StringIO in unicode_output_stream.
Further fix to unicode_output_stream.
Report filenames in upcall errors.

Match other errors, which report filenames due to having exceptions
associated with them. Since these errors don't stem from exceptions,
it's harder to directly follow where the problem came from.
Merge pull request #10 from emonty/master

Add filenames to upcall errors
Release 0.9.29
Opening 0.9.30 for development.