Fix MapBinder's duplicate key error message so that more than one binding with the same source gets listed twice.  This was causing a problem with the tests that turned stack traces off, because the two different bindings line numbers were both changed to "Unknown Source", so it collapsed them into a single item.

Maven ran all the flag variations for core + extensions.
Internal builds ran them all just for core, not the extensions (fixed in this CL).
Ant runs them all for core, but doesn't even run any extension tests AFAICT (not fixed in this CL).
Created by MOE:
1 file changed