Detect when an external command crashes

Detect when an external command crashes

This fixes a situation where LyX did not detect that something went
wrong (that an external comman crashed) and reported that export was
successful. To reproduce, use the following version of LuaTeX (the
bug in LuaTeX causing the crash has since been fixed):
LuaTeX, Version beta-0.79.1 (TeX Live 2014) (rev 4971)
Then open FeynmanDiagrams.lyx and export with PDF (LuaTeX).

In the documentation [1] for QProcess::exitCode() it states:
"This value is not valid unless exitStatus() returns NormalExit."

For more information, see:
https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg185317.html

[1] http://doc.qt.io/qt-5/qprocess.html#exitCode