ovsdb-idl: log error in client when db schema does not exist on server

ovsdb-idl: log error in client when db schema does not exist on server

A common error scenario with OVN is to attempt to use ovn-nbctl when
the OVN databases have not been created in ovsdb-server:
   1. ovn-nbctl sends "get_schema" request for OVN db to ovsdb-server.
   2. ovsdb-server fails to find requested db, sends error response
      to ovn-nbctl.
   3. ovn-nbctl receives the error response in ovsdb_idl_run(), but
      takes no specific action.
   4. ovn-nbctl hangs forever in IDL_S_SCHEMA_REQUESTED state (assuming
      a timeout wasn't requested on the command line).

Add a log message to inform the user of this situation.

Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>