Change a warning to an error if missing include

Change a warning to an error if missing include

If an included file is not found, it seems impossible that the final
output would be correct. Before this commit, when exporting on the
command line, LyX would create the PDF and exit with a zero error
code so unless the user was monitoring the terminal output, they
would not know there is a problem.

This change is consistent with 1a374a93.