process-lines

[Eli Zaretskii]

This function signals an error if the program it runs exits with a
non-zero status.  That sounds like an unfortunate limitation for a
general-purpose subroutine, since some programs convey useful
information through non-zero status.  Diff is one example of such
programs; there are others.

I suggest to add an optional argument to control the behavior when the
program exits "abnormally".