Raise HTTP errors if they are not API errors.

When we're unable to parse an XML API error from the response body,
check to see if the response had an HTTP error code, and if so,
raise the HTTP error instead of raising a ParseError.
Add .encode() call for Py3 compat
Adjust XML parse error types for 2.6 compat
Use {0} instead of {} for Py2.6 compat
Merge pull request #181 from ayust/raise_http_errors

Raise HTTP errors if they are not API errors.