Invalid enum values would previously result in a Sequel::DatabaseError
(wrapping PG::InvalidTextRepresentation) when saving. This changes
the behavior from an unhandled exception 500 error to a nice 400 error.
We previously had manually validations in LoadBalancer for the enum
values. This removes those. Only change in the tests is to an error
message, and I think the new error message is nicer.
In order for this to work, the Database needs to use the pg_enum plugin,
so this uses that and regenerates the schema cache with it.
cf0d7c1c46