[IMMPR] Do not raise a RuntimeError if user-config.py is missing

config2.py raises a RuntimeError if user-config.py is required but
not present. This looks ugly. Especially if operators are setting up
their bot they expect a pleasant hint instead of a traceback. And
developers usually know about this issue and do not need the code
snippet for debugging.

Change-Id: I15f7a6f356408e7113362745e81ec7758e9a01ab