Include traceback when wrapping import errors

[wolever]

Without the complete tracebacks it's fairly frustrating to try and track down the import errors.

[wolever]

Note: I haven't yet tested the multiprocessing and jinja2 handlers. They look like they should work, though, and if this patch is going to be accepted I'll test them properly.

[bitprophet]

I actually think I do display the traceback for the multiprocessing import, for this very reason, but it looks like that was not considered for those other optional requirements. Good catch!