Special care is needed so that we get an error message if the
file failed to parse, but not when it is missing. To avoid duplicating
the same error check in every caller, add an additional 'warn' boolean
to tell config_parse whether a message should be issued.
This makes things both shorter and more robust wrt. to error reporting.
This works around the goa daemon crash:
Jan 25 20:54:24 lon goa[1363]: goa-daemon version 3.10.2 starting [main.c:117, main()]
Jan 25 20:54:24 lon systemd[424]: Starting Legacy D-Bus Protocol Compatibility Daemon (PID 1363/UID 2702)...
Jan 25 20:54:24 lon systemd[424]: Started Legacy D-Bus Protocol Compatibility Daemon (PID 1363/UID 2702).
Jan 25 20:54:24 lon kernel: goa-daemon[1363]: segfault at 20 ip 00007f46914b26d5 sp 00007fff1ae6d9a0 error 4 in libtelepathy-glib.so.0.80.1[7f469144f000+228000]
Jan 25 20:54:24 lon systemd-coredump[1368]: Process 1363 (goa-daemon) dumped core.
Jan 25 20:54:32 lon goa[1375]: goa-daemon version 3.10.2 starting [main.c:117, main()]