ucm run.compiled hits an "unexpected exception" when told to run a non-existent file

[fwip]

To reproduce:

$ ucm run.compiled file-that-doesnt-exist
Uh oh, an unexpected exception brought the process down! That should never happen. Please file a bug report.

Here's a stringy rendering of the exception:

  file-that-doesnt-exist: openBinaryFile: does not exist (No such file or directory)

This case should probably have a better error message.