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.