Add Mutex in HttpError to make Error sync again

[djmcgill]

Closes #362 and provides a significant usability boost when using failure.

This is an API breaking change so will need to wait for a major version bump. If you'd rather I place it on the 0.6.x branch I can do that instead.

[troiganto]

I just found this PR because I stumbled over #362 myself. I looked a bit into this and ultimately, the error is not Sync because the hyper::client::Response contained in HttpError isn't. This is because reading the payload of the response is not thread-safe.

In my case, if such an error occurs, I'd like to print the reponse with status code and the response body. The solution in this PR, wrapping the entire struct in a mutex, would make this difficult because I'd be able to read the response only once, and it's gone afterwards.

What I'm wondering is: Would it make more sense to read the entire response into one or more Strings and include that in the error type? The type would then roughly look like this:

#[derive(Debug)]
pub enum Error {
    // Maybe include Headers and HttpVersion, too.
    UnsuccessfulRequest{ status: StatusCode, url: Url, body: String, },
    RateLimitI64,
    RateLimitUtf8,
}

I could hack together a PR for this, but I wanted to check first if there's any interest for that.

[Lakelezz]

Thanks for your pull request, but #532 ended up being a better solution to this problem!