Database: remove Table.find_exn

This was confusing because we already had {Row,Table,TableSet}.find
which threw exceptions. The only difference was that Table.find_exn
would throw a DBCache exception rather than Not_found.

We revert all callers to Table.find, and expect them (in the DB_ACCESS
layer) to convert the Not_found into the DB_ACCESS-specific exception
themselves.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>