oauth2: add error type for unsuccessful token endpoint status

Allows the HTTP response and body to be extracted without parsing
the error string, but keeps backwards compatibility for users who
are currently doing so.

Fixes golang/oauth2#173

Change-Id: Id7709da827a155299b047f0bcb74aa8f91b01e96
Reviewed-on: https://go-review.googlesource.com/84156
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

clientcredentials/clientcredentials.go

@@ -92,6 +92,9 @@ func (c *tokenSource) Token() (*oauth2.Token, error) {
 	}
 	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v)
 	if err != nil {
+		if rErr, ok := err.(*internal.RetrieveError); ok {
+			return nil, (*oauth2.RetrieveError)(rErr)
+		}
 		return nil, err
 	}
 	t := &oauth2.Token{