errors: Add exception for RAPI testing utilities

This exception is raised to abort before actually sending a LUXI call
(there is no LUXI server involved in the test). The testing utilities
catch the exception to report a success (i.e. the code didn't throw
an exception before due to invalid types, etc.). To allow the exception
to be thrown all the way to the test utilities, the HTTP server library
must ignore it. Also some overly generic exception handling is removed
from the RAPI request handler.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>