Output file from (xe|lua)latex is PDF, not DVI

Output file from (xe|lua)latex is PDF, not DVI

A few parts of our code depend on correctly identifying the output
format of LaTeX commands. One specific bug is that because the
output file was not correctly set, it was not removed after an
error. For example, this commit fixes the following bug:

1. Create a new document that contains "hello\blah" where \blah is
in an ERT box.
2. Compile with PDF (LuaTeX). You'll get an error because of \blah.
3. Close the error dialog.
4. Remove the text "hello" and compile again with PDF (LuaTeX).

The error dialog is shown and the "Show Output Anyway" button is
enabled. If you click it, it shows the previously compiled PDF (with
the text "hello"). With this commit, the button is correctly
disabled (and the output file is deleted).