`ErrorResponse#to_s` is highlighted

Exception#full_message that is used as ErrorReponse.to_s is highlighted by default, which is hardly desirable in most environments:

> p response.to_s
"/home/ojab/src/httpx/lib/httpx/response.rb:319:in `full_message': \e[1mTimed out (\e[1;4mHTTPX::TimeoutError\e[m\e[1m)\e[m\n"

while it could be disabled by hightlight kwargs, IMHO it would be more clear to return empty string, because no body was received. Also having whole backtrace in the message is awkward and makes body non-determenistic for testing purposes, for example.

I could craft the PR to (in order of my preference)

1. Replace it with ''
2. Make it #inspect (probably the best way to get human-readable exception, also no backtrace)
3. Make it highlight: false