Provide function to write RPC errors from HTTP middleware

[akshayjshah]

Is your feature request related to a problem? Please describe.
Even though Connect handlers work with net/http middleware, it's often necessary to re-implement existing HTTP middleware as Connect interceptors so that clients get nicely-annotated errors. This is a little silly.

Describe the solution you'd like
It'd be nice for Connect to include a function that HTTP middleware can use to encode Connect errors. func WriteError(http.ResponseWriter, *http.Request, error) might work.

Describe alternatives you've considered
The status quo is okay, but forces users to jump through more hoops that we'd like.

[anhnmt]

I look forward to this feature coming soon

[akshayjshah]

Released in v0.5.0!