Use the generic RequestLimitExceeded exception when possible

Rather than having lots of specific exceptions that all pertain to request
limitation, we can just use the RequestLimitExceeded exception when applicable,
which can also simplify the handling of exception mapping in fault.py.

Change-Id: I61b1e59912087fad286e74c22af15ffe6382bb65
Merge "Use the generic RequestLimitExceeded exception when possible"