Fix pattern match exhaustivity warnings
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 <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

djs55/ocaml-xenstore-server,mirage/ocaml-xenstore-server,mirage/ocaml-xenstore-server