Teach CTest what a ninja error looks like.

This will allow ninja dashboards to show ninja errors as well
as compiler errors from the ninja build.

CTestCustom.cmake.in

@@ -1,3 +1,7 @@
+SET(CTEST_CUSTOM_ERROR_MATCH
+  ${CTEST_CUSTOM_ERROR_MATCH}
+  "ERROR:")
+
 SET(CTEST_CUSTOM_WARNING_EXCEPTION
   ${CTEST_CUSTOM_WARNING_EXCEPTION}
   "xtree.[0-9]+. : warning C4702: unreachable code"