Convert iallocator_runner rpc to new result style

This patch converts this rpc into the new style. Since the function
already had some error handling, we remove this custom error reporting
and replace it with our (new-style) result type. This allows significant
cleaning up of this code.

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