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!

(cherry picked from commit d4dba427acbf6f0c4f17343b6431c2e461110810)