fix: add missing return statements in error handler paths

[jet-go]

References to other Issues or PRs

N/A

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

Add missing return after handling error to avoid modifying response headers after response is sent which causes error http: superfluous response.WriteHeader.

Other comments

This issue only happens with non-legacy unescape modes. The decision of whether % should be actually escaped or not depends on the escape mode & is not fixed/implemented in this PR.

[johanbrandhorst]

LGTM!

[johanbrandhorst]

Looks like tests are failing under Bazel, for some reason:

==================== Test output for //runtime:runtime_test:
--- FAIL: TestMuxServeHTTP (0.00s)
    --- FAIL: TestMuxServeHTTP/28 (0.00s)
        mux_test.go:620: w.Body = "{\"code\":2, \"message\":\"malformed path escape \\\"%\\\"\", \"details\":[]}"; want "{\"code\":2,\"message\":\"malformed path escape \\\"%\\\"\",\"details\":[]}"; patterns=[{GET [2 0 1 0 4 1] [foo id] }]; req=&{GET https://host.example/foo/%25 HTTP/1.1 1 1 map[Content-Type:[application/json]] {} 0x709ac0 0 [] false host.example map[] map[] <nil> map[]   <nil> <nil> <nil>  {{}} <nil> [] map[]}

[jet-go]

fixed

[johanbrandhorst]

Thanks for your contribution!