Ensure that exception prints correct text.

Fixes bug 1095333

Change-Id: I5077108de8dacaab4bcf0d7236123f960676d591
Set default core_plugin to None

Fixes bug 1093989

Change-Id: Ib0b77994b813e6b2ec0621949371c66d66472896
Merge "Ensure that exception prints correct text."
Merge "Set default core_plugin to None"