Don't log traceback on expected console error.

When requesting a console before an instance is read (while still
building) we will get an InstanceNotReady exception.  This exception is
raised in the compute manager.  The explanation in the exception will
include the traceback from the manager side.  If we use this as the
explanation, we end up with a traceback in the API log, even though it's
just an INFO log message.

Use a more basic explanation, which is accuately more consistent with
other cases of raising HTTP exceptions, which doesn't include a
traceback.

Fix bug 1154327.

Change-Id: Idca443165983289a0df40970f68f95d5550ce72a
Merge "Don't log traceback on expected console error."