httputil : wrap handlers for additional context

[desimone]

Summary

1. We extend standard library's http.HandleFunc to support errors. This improves (imho) request flow to be more explicit and safer. In particular, returning an error and not another request handler func ensures that we don't accidentally forget to return from the parent func after a error request handler is called.

For example

...
		httputil.ErrorResponse(w, r, err)
		return // request will actually fall through if this return is omitted!
	}
fmt.Print("some super duper secret info")
}

vs

...
		return err
	}
fmt.Print("some super duper secret info")
}

In practice, this mostly meant replacing http.HandlerFunc with our own httputil.HandlerFunc and changing our handler function definitions to return an error type.

2. We create a custom HTTP error type that allows us uniformly handle pomerium http error, and include additional request context where available like original request url (Improve user experience when authenticate callback session expires #383).

Related issues

• Fixes httputil/errors: http template should not instantiate on each call #395
• Fixes Improve user experience when authenticate callback session expires #383

See also

• https://blog.questionable.services/article/http-handler-error-handling-revisited/
• https://blog.questionable.services/article/custom-handlers-avoiding-globals/

Screenshots

Checklist:

• add related issues
• updated unit tests
• ready for review

[codecov]

Codecov Report

Merging #413 into master will decrease coverage by 0.2%.
The diff coverage is 94.4%.

@@           Coverage Diff            @@
##           master    #413     +/-   ##
========================================
- Coverage    87.8%   87.6%   -0.3%     
========================================
  Files          52      52             
  Lines        2518    2482     -36     
========================================
- Hits         2212    2175     -37     
- Misses        240     241      +1     
  Partials       66      66

Impacted Files	Coverage Δ
internal/httputil/router.go	33.3% <0%> (-66.7%)	⬇️
proxy/proxy.go	96.6% <100%> (ø)	⬆️
proxy/middleware.go	100% <100%> (ø)	⬆️
authenticate/handlers.go	95.1% <100%> (+0.2%)	⬆️
internal/middleware/middleware.go	100% <100%> (ø)	⬆️
internal/httputil/handlers.go	100% <100%> (ø)	⬆️
internal/urlutil/url.go	100% <100%> (ø)	⬆️
proxy/handlers.go	100% <100%> (ø)	⬆️
internal/urlutil/signed.go	93.7% <100%> (ø)	⬆️
internal/httputil/errors.go	87.5% <92.5%> (-3.5%)	⬇️
... and 5 more

[travisgroth]

One comment but LGTM