Improvement: change undefined object error in [REPORT] section to a warning

[MdoubleDash]

In the event that an entity (subcatchment, node, or link) within the reporting section cannot be located within the model, SWMM promptly issues an error message:

ERROR 209: undefined object Test_Element_01 at line 5577 of [REPORT] section:
  SUBCATCHMENTS object Test_Element_01

Dealing with this can be a real pain, especially when you're juggling multiple scenarios. So, instead of freaking out with an error, it would be way cooler if SWMM just gave a heads-up:

WARNING 13: undefined object for reporting Test_Element_01

[cbuahin]

It will be nice to make the model output a self contained unit that is independent from an input file, but your suggestion is a good intermediate step.