[LinuxCNC/linuxcnc Issue#308] AXIS/etc show all gcode errors as ‘Unable to open file <>‘

Here are the steps I follow to reproduce the issue:

1. Put any bad gcode in a gcode file
2. open it with AXIS

This is what I expected to happen:

AXIS shows the real error

This is what happened instead:

AXIS shows the error ‘Unable to open file <>‘

It worked properly before this:

broken in 2.7.10, OK in 2.7.9. Almost certainly broken by #299.

Reported by @andypugh via the mailing list, reproduced by me