Remove tables from metadata when autoload fails

If autoloading of a table fails, don't register it in a metadata
instance. It seems that the original behaviour was accidentally
changed in f6198d9abf453182f4b111e0579a7a4ef1614e79, restore it.

Closes issue #2988
Merge branch 'fix_table_registration' of https://github.com/malor/sqlalchemy into t
changelog to fix #2988