A failed table autoload hampers subsequent definition of the same table

When an autoload of a non-existing table fails:

>>> from sqlalchemy import *
>>> md = BoundMetaData('sqlite:///test.db')
>>> Table('nonesuch', md, autoload=True)
Traceback (most recent call last):
...
sqlalchemy.exceptions.NoSuchTableError: nonesuch

the table remains in the metadata dict:

>>> md.tables
{'nonesuch': Table('nonesuch',
BoundMetaData(),
schema=None)}

that way, a subsequent definition of the same table fails:

>>> Table('nonesuch', md,
...     Column('id', Integer, primary_key=True))
Traceback (most recent call last):
...
sqlalchemy.exceptions.ArgumentError: Table 'None.nonesuch' is already
defined. specify 'redefine=True' to remap columns, or 'useexisting=True'
to use the existing table

One could use the 'redefine=True' parameter to avoid this error, but I would prefer avoiding it, because in some cases that would actually, and wrongly, redefine a table.

The attached patch deregisters the table, but only when autoloading raises NoSuchTableError, and therefore makes the subsequent definition work.

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