When altering a partitioned table and adding an incorrectly specified
partition, an assertion was hit rather than gracefully erroring out.
Make sure that the requested partition matches the underlying table
definition before continuing down into the altering code. This also
adds a testcase for this.

Reported-by: Kalen Krempely in #6967
Reviewed-by: NPaul Guo <pguo@pivotal.io>
Reviewed-by: NGeorgios Kokolatos <gkokolatos@pivotal.io>