Improve error-handling for non-JSON response

[TomOnTime]

Frequently I get an error from the library as follows:

Fail to decode the response body (error 'invalid character '<' looking for beginning of value')

I haven't looked deeply into the code but it looks like there are some errors that return a textual error message, not a JSON blob to be parsed. The code seems to try to parse it as JSON anyway and thus returns the above error.

I bet there's an error code or something that can be used to determine that the payload isn't JSON and should just be used as the error string.