Client attempts to parse the body of every error it receives as JSON
regardless of the content-type. This commit rectifies by only parsing
he error body as JSON if the Content-Type header is set to
either "application/json" or "application/vnd.api+json".
Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>
If this slice ends up empty after parsing the HTTP response body, it
means the body is not well-formed. We've probably encountered an error
message produced by something that uses a different JSON schema, or
an error that just happens to validate as JSON.
An empty errcode.Errors slice is not a very useful thing to return,
since its Error() output is just `<nil>`. Detect this case, and instend
return an UnexpectedHTTPResponseError.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>