When a file plug-in fails to open a file, it usually displays an error
message before exiting.  After that, the GIMP pops up a second window
with another error message such as: "GIMP Message / Opening 'testfile'
failed: Plug-in could not open image".  This second message is
redundant if the plug-in has already popped up its own error window.

To reproduce: "touch empty.png empty.jpg empty.gif" and then try to
load these files.  If the plug-in crashes or exits incorrectly (see
bug #139571), you also get two error messages.

Besides the codes returned by the file plug-ins to indicate success or
failure, we may need to use a special code meaning: "I failed but I
already gave a detailled report to the user about it so please
propagate the error upstream silently."

Alternatively, we could leave the exit codes as they are now and
simply remove the error message "Opening 'testfile' failed: Plug-in
could not open image" and ensure that all error messages from the
plug-ins mention which file could not be opened.  It would also be
nice to modify plug_in_recv_message() in app/plug-in/plug-in.c and
improve the message displayed when a file plug-in has crashed so that
it displays the name of the file that could not be loaded or saved,
but I don't know how this information could be accessed in that
function.

(Note: I thought that a similar bug had been reported earlier, but I
could not find it.)