Fix pattern match exhaustivity warnings

Although the function names aren't obvious, these were both on the
request marshalling path. These functions are only used by our
well-behaved request marshaller, which should never marshal either
Errors or Watchevents as requests.

Fixed by making one function total, and the other an explicit exception
and a note describing why it can never happen. Perhaps we could
encode this in the types somehow.

Fixes [djs55/ocaml-xenstore#13]

Signed-off-by: David Scott <dave.scott@eu.citrix.com>