Both lyx2lyx and LyX write output to the terminal, and it's helpful
to know which messages are coming from lyx2lyx. For example, before
this commit if we opened a document in LyX that has a newer file format than
lyx2lyx can deal with, we see the following output in the terminal:
Warning: 619: Format not supported.
Warning: Quitting.
Error: Document format failure
The first two lines (the warnings) are output from lyx2lyx, and the
third is output by LyX. This output was particularly confusing
because I first thought "LyX tries to quit if the document is too
new?", but in fact LyX still stays open; it just doesn't open the
document. After this change, the output is now the following:
lyx2lyx warning: 619: Format not supported.
lyx2lyx warning: Quitting.
Error: Document format failure