Make invalid dialects raise a sensible exception

[Former user]

Misspelling a dialect produces an ImportError; some form of "No support for dialect" error would be more useful.

Example code (misspelled sqlite):

from sqlalchemy import create_engine
create_engine('qlite:///:memory:')

[Mike Bayer]

fine for the sprints

[Former user]

Should result in the following post-patch:

sqlalchemy.exc.ArgumentError: Could not determine dialect for 'qlite'.

[Mike Bayer]

Removing milestone: 0.7.0 (automated comment)