provide more useful info to help the user debug template syntax errors

The template filename, line number, and error message are now displayed.
provide more information when a plugin barfs an exception

This is still not very useful because the stack trace is in the
context of the re-raise by futures.Future, not within the thread
where it was originally raised.  I'm not sure whether it's possible
to fix this without using a different thread handling mechanism.

See also issue #6.
Merge pull request #20 from aspiers/error-handling

Try to make it easier to debug issues with plugins and templates