This avoids warning 52 in OCaml code such as:
try URI.parse_uri arg
with Invalid_argument "URI.parse_uri" -> ...
which prints:
Warning 52: Code should not depend on the actual values of
this constructor's arguments. They are only for information
and may change in future versions. (See manual section 8.5)
In the long term we need to change fish/uri.c so that we can throw
proper errors.
0970bd0608