Fix most of bug 5539.

Fix most of bug 5539.

If a file fails to load, we do not try to load it again. The downside is that, even if you fixed the problem, then LyX won't try to load it again in that session. I tried resetting the failedtoload_ variable in fileChanged(), but that didn't work, as it doesn't seem to get hit. I'm not sure what to do here, but it isn't a huge issue.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@27688 a592a061-630c-0410-9148-cb99ea01b6c8