Batchmapper: better error when system code is missing

[martijn]

When trying to map to a system code that is not supported, a good error message should be given instead of just returning an empty result.

This is particularly important for 'En', which is deprecated, but this is not obvious.

See also this discussion:  http://groups.google.com/group/bridgedb-discuss/browse_thread/thread/11147cbc4b56ffa0