A number of SQLAlchemy exception are printed by using their message
attribute. However PEP-352 deprecates this attribute from Python 2.6
onwards and drop it for Python 3 and later versions.
This commit replaces printing the message attribute by printing the
exception itself, relying on the str method. This is actually
equivalent for exceptions with only a single argument (as required for
all exceptions on Python 3) as can be seen by the message property
getter in the aforementioned PEP.