tailscale: read ACL validation errors from a 200 OK response

Per [docs](https://github.com/tailscale/tailscale/blob/main/api.md):

> The HTTP status code will be '200' if the request was well formed and
> there were no server errors, even in the case of failing tests or an
> invalid ACL. Look at the response body to determine whether there was a
> problem within your ACL or tests:

Updates tailscale/terraform-provider-tailscale#227

tailscale/client.go

@@ -441,7 +441,14 @@ func (c *Client) ValidateACL(ctx context.Context, acl ACL) error {
 		return err
 	}
 
-	return c.performRequest(req, nil)
+	var response APIError
+	if err := c.performRequest(req, &response); err != nil {
+		return err
+	}
+	if response.Message != "" {
+		return fmt.Errorf("ACL validation failed: %s; %v", response.Message, response.Data)
+	}
+	return nil
 }
 
 type DNSPreferences struct {

tailscale/client_test.go

@@ -950,7 +950,36 @@ func TestClient_ValidateACL(t *testing.T) {
 	assert.NoError(t, err)
 	assert.EqualValues(t, server.ResponseBody, acl)
 	assert.EqualValues(t, http.MethodPost, server.Method)
+	assert.EqualValues(t, "application/json", server.Header.Get("Content-Type"))
 	assert.EqualValues(t, "/api/v2/tailnet/example.com/acl/validate", server.Path)
+
+	tests := []struct {
+		name         string
+		responseCode int
+		responseBody any
+		wantErr      string
+	}{
+		{
+			name:         "403_response",
+			responseCode: 403,
+			responseBody: tailscale.APIError{Message: "access denied"},
+			wantErr:      "access denied",
+		},
+		{
+			name:         "200_response_with_error",
+			responseCode: 200,
+			responseBody: tailscale.APIError{Message: "validation failed"},
+			wantErr:      "validation failed",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			server.ResponseCode = tt.responseCode
+			server.ResponseBody = tt.responseBody
+			err := client.ValidateACL(context.Background(), acl)
+			assert.ErrorContains(t, err, tt.wantErr)
+		})
+	}
 }
 
 func TestClient_UserAgent(t *testing.T) {