Return error from twirp.WriteError

[ofpiyush]

Moving discussion from the PR as @spenczar requested.

Recap:

Current implementation of twirp.WriteError does not return any error occurred while writing the TwirpError.

Humor

WriteError does not return the error if an error occurs while writing the error.

Since the public API currently returns nothing, all existing code looks like

twirp.WriteError(resp, err)

As no return is being processed, adding a return to this method will not break existing code.

[spenczar]

You're right that it won't break most code. The only exception would be that the function's type changes, so if someone has a var w func(*http.ResponseWriter, error) = twirp.WriteError, that code will break. That seems pretty unusual, and the function is new enough that it might be safe to modify without any significant changes.

It's a bit of a violation of semver to make this part of a minor release, but I think it might be a forgivable one.

[spenczar]

Oh, and in case it isn't obvious - yes, it should return an error if it fails to write.