Do not attempt to output part raw if part is not GMimePart.

This was a minor oversite in checking of part type when outputing
content raw.  This was causing gmime was to throw an exception to
stderr.

Unfortunately the gmime exception was not being caught by notmuch, or
the test suite.  I'm not sure if notmuch should have done anything in
this case, but certainly the test suite should be capable of detecting
that something unexpected was output to stderr.