switch to go vendoring

vendor/github.com/crewjam/httperr/handler.go

@@ -0,0 +1,20 @@
+package httperr
+
+import "net/http"
+
+// The HandlerFunc type is an adapter to allow the use of
+// ordinary functions as HTTP handlers.  If f is a function
+// with the appropriate signature, HandlerFunc(f) is a
+// Handler object that calls f.
+type HandlerFunc func(http.ResponseWriter, *http.Request) error
+
+// ServeHTTP calls f(w, r).
+func (f HandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if err := f(w, r); err != nil {
+		if v := r.Context().Value(onErrorIndex); v != nil {
+			v.(func(error))(err)
+		} else {
+			Write(w, r, err)
+		}
+	}
+}