Tweak the error message for foreign key indices

"requires an index" -> "requires a unique index" in the error message for foreign keys on columns that are not (uniquely) indexed
Update test-foreign-key-ddl.yaml with updated error message on FK indices
merge louisrli: Tweak the error message for foreign key indices

Merge pull request #1172 from louisrli/sql-layer/patch-1
https://github.com/FoundationDB/sql-layer/pull/1172