ServerError: Default empty errBody to errReasonPhrase

Currently if one doesn't specify `errBody` when calling `err401` for example, the browser will return an empty response.

This is a very bad default since it gives a bad user experience.

I think the best thing to do would be to just use `errReasonPhrase`, although that might lead to problems with content-type mismatches. At least the error message will be in response.

The alternative is to make errBody a required field, which is backwards incompatible.