Improve some exceptions messages

[eMerzh]

Hi,

thanks for your project,
i recently switched fro python-mysql to pymysql using sqlalchemy.
and some of my exception had loose some really helpfull details.

like before :

InvalidRequestError("This Session's transaction has been rolled back due to a previous exception 
during flush. To begin a new transaction with this Session, first issue Session.rollback(). Original
exception was: UPDATE statement on table 'XXXX' expected to update 1 row(s); 0 were matched.",)

After:

InvalidRequestError("This Session's transaction has been rolled back due to a previous exception
during flush. To begin a new transaction with this Session, first issue Session.rollback(). Original 
exception was: 'ProtocolError' object has no attribute 'translate'",)

it would be really helpfull to have those details back :)

Many Thanks

[methane]

Complete step to reproduce? (hopefully, without SQLAlchemy).

[methane]

At least, full traceback is required.
I can't understand anything from your report.