Implement shorthand error handling

[jnichols-git]

RFC 7807 defines a standard response format for HTTP handling problems. Error handling in Go has always been a bit verbose, but HTTP introduces the additional step of constructing and writing a response each time, which can be very frustrating if you want to have an error that isn't 500 Internal Server Error. I would like to implement a standard ProblemResponse that takes in a ResponseWriter and fields related to 7807 (type string, title string, status code, detail string, instance string) and writes a formatted response with content type `application/problem+json. This function/type should validate that the inputs are OK according to spec (so type and instance must be URIs, etc).

Notably, this should not write a problem on panics--exposing internal errors can be a serious security issue, and the decision to use RFC 7807 or not should be a conscious one made during regular operation.

Since 7807 is a proposal, I would also like to implement an ErrorResponse that just takes in a code and an error instead of the problem fields.

[SuneelFreimuth]

Down to take this.

[jnichols-git]

Assigned!