Improve error message for missing endpoint

Horizon logs an error when it wants an endpoint
that the client can't find, but the error does not
say which endpoint is missing. This change includes
more details in the error message to make debugging
from the horizon logs easier.

Change-Id: I6f0aacd469500238f13a2178a220268d86d31fe5
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>