Bug: a malformed XSTR checker in FRED

[jlberman]

When a malformed XSTR is in a briefing (for instance), it throws an error that shows "Alpha 1 is not present" in a mission, and the error checker doesn't point to the correct issue, which is that the XSTR is malformed. Just to give an example, I deliberately set up a malformed XSTR in line 86...

...but the FRED error message points to line 404 of the mission.

[Goober5000]

The XSTR warning did fire, but only in a debug build. But both @jlberman and I think there's a case to be made for displaying this warning in release, so I'll make a PR.