Unsupported scheme and empty URL exit 92 though no request was attempted

[korya]

Problem

A URL with an unsupported scheme, or an empty URL, exits 92 ("the request produced no usable response") even though no request was ever attempted — Go rejects the scheme in the client, before any socket is opened.

Reproduction

$ http-assert --assert-ok ftp://example.com/x
Error: Cannot perform request: failed to send request:
- Get "ftp://example.com/x": unsupported protocol scheme "ftp"
$ echo $?
92

$ http-assert --assert-ok ""
Error: Cannot perform request: failed to send request:
- Get "": unsupported protocol scheme ""
$ echo $?
92

Why it matters

--help defines the codes as:

71   the invocation was rejected; no request was attempted
92   the request produced no usable response

Both cases meet the definition of 71 exactly. Reporting them as 92 tells a CI job the network or the service is at fault when the caller mistyped a URL — the confusion #86 was written to remove.

For contrast, a scheme-less URL already gets this right:

$ http-assert --assert-ok 127.0.0.1:8099/ok
Error: Cannot create request 'GET 127.0.0.1:8099/ok': parse "127.0.0.1:8099/ok": first path segment in URL cannot contain colon
$ echo $?
71

Other

The empty-URL path also renders poorly: FAILED: GET (HTTP/1.1) (two spaces) with an empty Host: in the dump.

Suggested fix

Validate the scheme when the request is built, alongside the existing URL parse check, so both exit 71.