Display more user friendly error for invalid os logins in Web UI

Host resolution no longer happens prior to connections in the Web
UI and all dial attempts are by UUID. When an invalid login is
attempted the error message constructed only contained the info
provided to the dial request: a login and a Host UUID. To make
this error more user friendly access denied errors are now
augmented with the hostname if the user has permissions to that
host and the error occurs only due to an invalid login.

Closes #23719

lib/web/apiserver_test.go

@@ -7790,6 +7790,10 @@ type authProviderMock struct {
 	server types.ServerV2
 }
 
+func (mock authProviderMock) ListUnifiedResources(ctx context.Context, req *authproto.ListUnifiedResourcesRequest) (*authproto.ListUnifiedResourcesResponse, error) {
+	return nil, nil
+}
+
 func (mock authProviderMock) GetNode(ctx context.Context, namespace, name string) (types.Server, error) {
 	return &mock.server, nil
 }

lib/web/terminal.go

@@ -105,6 +105,7 @@ type UserAuthClient interface {
 	CreateAuthenticateChallenge(ctx context.Context, req *authproto.CreateAuthenticateChallengeRequest) (*authproto.MFAAuthenticateChallenge, error)
 	GenerateUserCerts(ctx context.Context, req authproto.UserCertsRequest) (*authproto.Certs, error)
 	MaintainSessionPresence(ctx context.Context) (authproto.AuthService_MaintainSessionPresenceClient, error)
+	ListUnifiedResources(ctx context.Context, req *authproto.ListUnifiedResourcesRequest) (*authproto.ListUnifiedResourcesResponse, error)
 }
 
 // NewTerminal creates a web-based terminal based on WebSockets and returns a
@@ -912,6 +913,21 @@ func (t *sshBaseHandler) connectToNode(ctx context.Context, ws terminal.WSConn,
 		// The close error is ignored instead of using [trace.NewAggregate] because
 		// aggregate errors do not allow error inspection with things like [trace.IsAccessDenied].
 		_ = conn.Close()
+
+		// Since connection attempts are made via UUID and not hostname, any access denied errors
+		// will not contain the resolved host address. To provide an easier troubleshooting experience
+		// for users, attempt to resolve the hostname of the server and augment the error message with it.
+		if trace.IsAccessDenied(err) {
+			if resp, err := t.userAuthClient.ListUnifiedResources(ctx, &authproto.ListUnifiedResourcesRequest{
+				SortBy:              types.SortBy{Field: types.ResourceKind},
+				Kinds:               []string{types.KindNode},
+				Limit:               1,
+				PredicateExpression: fmt.Sprintf(`resource.metadata.name == "%s"`, t.sessionData.ServerID),
+			}); err == nil && len(resp.Resources) > 0 {
+				return nil, trace.AccessDenied("access denied to %q connecting to %v", sshConfig.User, resp.Resources[0].GetNode().GetHostname())
+			}
+		}
+
 		return nil, trace.Wrap(err)
 	}