app: do not crash when opening a file with unimplemented compression

Our code was planning zlib and fractal compressions for eons, but would
crash against a file which would be actually using these. It means that
if we implement one of these compressions for 2.10, anyone with GIMP 2.8
(and likely earlier too) would crash when opening a legit file using the
new compressions. That's very bad.
Never use g_error() in non-fatale, expected, situations!
configure.ac, docs: use the correct gimpdir in the manual pages

It's now either $XDG_CONFIG_HOME/$gimpdir/2.9 or directly $gimpdir,
depending on whether $gimpdir is a relative or absolute path. Build
the resulting string in configure and put it into the manpage_gimprc
variable so it can be used in the manpage sources.