rapi: rework error handling

Currently the rapi code doesn't have any custom error handling; any
exceptions raised are simply converted into an HTTP 500 error, without
much explanation.

This patch adds a couple of generic SubmitJob/GetClient functions that
handle some errors specially so that they are transformed into HTTP
errors, with more detailed information.

With this patch, the behaviour of rapi when the queue is full or
drained, or when the master is down is more readable.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>