ovsdb: Fix formatting of ovs.db.Error on Python 2.6.

ovsdb: Fix formatting of ovs.db.Error on Python 2.6.

All of the negative Python OVSDB tests were failing on Python 2.6 because
"%s\n" % e yielded the empty string on that version of Python.  In turn,
that was because ovs.db.error.Error.__unicode__ was being called instead of
ovs.db.error.Error.__str__.  I'm puzzled why that was happening, but this
commit fixes it and also seems like a small code cleanup.

Peter Balland helped me gain some insight on this problem.

CC: Peter Balland <peter@nicira.com>
CC: Reid Price <reid@nicira.com>