Exit with error if child exits with error (#10188)

Exit with error if child exits with error (#10188)

Before, it could have been the case that

  lyx -e pdf2 file.lyx

had exit code 0 even though file.lyx includes a file that exits with
error. If compiled in the GUI a warning was given, but from the
command line exit code it would seem there was no problem. The exit
code for this case is now non-zero and the word "Warning" is now
removed from the message because it should be treated as an error.

An exception is thrown from InsetInclude and is caught in
Buffer::makeLaTeXFile() and added to the error list.

The (similar) use case at #8840 is also fixed by this commit.