Update tap authz error with doc URL (#3196)

When the Tap APIServer's SubjectAccessReview fails, return an error
message pointing the user to: https://linkerd.io/tap-rbac

Depends on linkerd/website#450
Part of #3191

Signed-off-by: Andrew Seigner <siggy@buoyant.io>

controller/tap/handlers.go

@@ -136,7 +136,7 @@ func (h *handler) handleTap(w http.ResponseWriter, req *http.Request, p httprout
 		req.Header[h.groupHeader],
 	)
 	if err != nil {
-		err = fmt.Errorf("SubjectAccessReview failed with: %s", err)
+		err = fmt.Errorf("tap authorization failed (%s), visit https://linkerd.io/tap-rbac for more information", err)
 		h.log.Error(err)
 		renderJSONError(w, err, http.StatusForbidden)
 		return

controller/tap/handlers_test.go

@@ -39,7 +39,7 @@ func TestHandleTap(t *testing.T) {
 			},
 			code:   http.StatusForbidden,
 			header: http.Header{"Content-Type": []string{"application/json"}},
-			body:   `{"error":"SubjectAccessReview failed with: not authorized to access namespaces.tap.linkerd.io"}`,
+			body:   `{"error":"tap authorization failed (not authorized to access namespaces.tap.linkerd.io), visit https://linkerd.io/tap-rbac for more information"}`,
 		},
 	}