Validate protocol regexp object earlier

[edgul]

One of the reasons PR 93 (#93) was needed is that compute_protocol_matches_special_scheme doesn't check for an error-free regexp object, as done with other component compilations that check optionally_transpose_regex_error, for example.

PR 93 prevents the crash, and the error will bubble up the same, but I think we can still catch this before getting as far as matching on the protocol (even though this is kind of a special case of needing to parse and match on the protocol so the parser can create an Init object correctly).