Implment encoding_errors for cx_oracle

Implment encoding_errors for cx_oracle

Added dialect-level flag ``encoding_errors`` to the cx_Oracle dialect,
which can be specified as part of :func:`.create_engine`.   This is passed
to SQLAlchemy's unicode decoding converter under Python 2, and to
cx_Oracle's ``cursor.var()`` object as the ``encodingErrors`` parameter
under Python 3, for the very unusual case that broken encodings are present
in the target database which cannot be fetched unless error handling is
relaxed.  The value is ultimately one of the Python "encoding errors"
parameters passed to ``decode()``.

Closes: #4801
Fixes: #4799
Change-Id: I1d542ba367bcd187347c54db1fee815f7890e71c