mdslink: crash while loading undefined instrument

[superctr]

Crashes with a C++ runtime error if an undefined instrument is referenced in MML input:

terminate called after throwing an instance of 'std::out_of_range'
  what():  map::at

This does not happen in mmlc, which will instead silently continue. I think the behavior should be the same in both programs: either mdslink should silently continue or we should throw InputError in both programs when this happens (I prefer this).