More informative error message on name collisions

This is the proposed fix for #2506. It provides a more complete message
with regards to name collisions including informing of the opposing
containers ID.

I have included a test to ensure that the correct short id is displayed
to make the message easier to understand.
Merge pull request #2638 from FreakyDazio/2506-name-conflict-error

More informative error message on name collisions