Update the DROP CONSTRAINT processing to allow dropping constraints by
name and have them work on GFK, FK, UNIQUE, and PKs. 

Add new error code for the drop constraint that does not exist.
Add missing error
merge tjoneslo: Update the DROP CONSTRAINT processing to allow dropping constraints by

Merge pull request #1109 from tjoneslo/sql-layer/drop-constraint-fixes
https://github.com/FoundationDB/sql-layer/pull/1109