horizon handles troveclient.ClientException as a recoverable error,
with troveclient defined via "from troveclient import exceptions
as troveclient". this patch imports oslo's apiclient exceptions
in troveclient/exceptions.py to ensure all ClientExceptions are
considered recoverable. also removes unused exceptions that were
unnecessarily ported from the compat version of the client. this
fix is necessary to avoid seeing the generic error message of
"Error: There was an error submitting the form. Please try again."
in horizon vs. the explicit error message.
Change-Id: I94c8ec0289821ccd365d6689488a3e25877c1560
Closes-Bug: #1292033
10ff1cf745