Prevent errors test fail if $GOPATH is something like /home/user/caddy (#2347)

caddyhttp/errors/errors.go

@@ -142,7 +142,7 @@ func (h ErrorHandler) recovery(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Trim file path
-	delim := "/caddy/"
+	delim := "/github.com/mholt/caddy/"
 	pkgPathPos := strings.Index(file, delim)
 	if pkgPathPos > -1 && len(file) > pkgPathPos+len(delim) {
 		file = file[pkgPathPos+len(delim):]