- fix test_single test to use default dialect

- fix test_single test to use default dialect
- The exception raised by Session when it is used
subsequent to a subtransaction rollback (which is what
happens when a flush fails in autocommit=False mode) has
now been reworded (this is the "inactive due to a
rollback in a subtransaction" message). In particular,
if the rollback was due to an exception during flush(),
the message states this is the case, and reiterates the
string form of the original exception that occurred
during flush. If the session is closed due to explicit
usage of subtransactions (not very common), the message
just states this is the case.

- The exception raised by Mapper when repeated requests to
its initialization are made after initialization already
failed no longer assumes the "hasattr" case, since
there's other scenarios in which this message gets
emitted, and the message also does not compound onto
itself multiple times - you get the same message for
each attempt at usage. The misnomer "compiles" is being
traded out for "initialize".