This question is very similar to [this issue][1], however I have a slight derivation.

Currently I am upserting Cases via the Data Loader, this way I can insert Cases and relate some look-up fields by their External Id.

The problem I'd like to solve is finding out if it is possible to do the following:

If a reference is found, proceed as normal. If the External Id doesn't match a value, save some other information.

Basically, is there a way to "DMLException when lookup's external id is unknown" from within a trigger?


  [1]: http://salesforce.stackexchange.com/questions/30876/prevent-dmlexception-when-lookups-external-id-is-unknown/30884#30884 "this"