driver/python: silient unfriendly tracebacks

Qutoes comments by coffeemug from issue #2098:

  I noticed from #2097 that some user errors are printed as Python
  exceptions (with Traceback and everything). They shouldn't be -- we
  should just print proper error messages and return user values. Printing
  Python exceptions and tracebacks is really unfriendly in those cases.

Link: https://github.com/rethinkdb/rethinkdb/issues/2098

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Merge pull request #2111 from Aleaxander/yliu-2098

driver/python: silient unfriendly tracebacks