Regression 10397 ICE with __error in concatenation

ErrorExp was allowing implicit conversions in some cases, because it is derived from IntegerExp! This made errors become un-errors again in some circumstances.

http://d.puremagic.com/issues/show_bug.cgi?id=10397
Better bug report:
http://d.puremagic.com/issues/show_bug.cgi?id=10412