should error if dbtype is unknown

[tbm]

I got a weird message that didn't make sense:

  File "/home/tbm/scratch/cvs/members/SPI.py", line 782, in get_stats
    cur = self.data['conn'].cursor()

until I figured out I used sqlite instead of sqlite3 for `dbtype.

Maybe something like:

        else:
            print("Unknown dbtype %s" % dbtype)

in __init__ of MemberDB.

(or maybe some error call instead of print)