interp: unwind with error if returning-to file cannot be reopened

this fixes #357 Milltask segfault when opening a missing file.
add a test of returning from o-word subroutine

This reproduces bug #357.
Merge remote-tracking branch 'origin/v2.5_branch'

Conflicts:
	src/emc/rs274ngc/interp_o_word.cc