Fix a couple problems in rbt post error handling.
Fix a couple problems in rbt post error handling.

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 `APIError`s 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.

Testing Done:
- Ran unit tests.
- Smoke tested rbt post error handling.

Fixes bug 4858.

Reviewed at https://reviews.reviewboard.org/r/11142/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools