Added two new exceptions for CloudSearch's document service

Currently, when the `commit()` method gets called on the document service it is hard to tell what's going based on the error that gets thrown. 

At the moment `CommitResponse` correctly looks up the results of the API call, but never actually returns the error to the user. Instead, right at the end of `CommitResponse`'s `__init__()` function, a check is done to ensure that the number of add/delete operations matches what was expected. This results in a `CommitMismatchError` getting thrown which doesn't contain the actual error that caused the mismatch in the first place.

This patch makes the error message available to the end user.