When rbt post encounters an error when creating the review request,

there were a couple problems that could manifest:

The code that attempted to handle the case of updating an existing

  review request on services that use changenums (perforce, plastic) was

  using an old variable name, causing a NameError.
The error message formatting for other types of APIErrors was

  passing the exception instance into force_unicode, which only

  accepts unicode or bytes. This has been changed to cast first to call

  the __str__ method.

Ran unit tests.
Smoke tested rbt post error handling.