Currently if any command errors out with G_IO_ERROR_NOT_SUPPORTED,
flatpak prints the usage information (the output of --help), but this is
not correct. For example if the create-usb command hits that error when
trying to use extended attributes on a filesystem that doesn't support
them, the help output is printed as we saw here:
https://github.com/flatpak/flatpak/issues/2019#issuecomment-416798304
So this commit removes the check for G_IO_ERROR_NOT_SUPPORTED in
flatpak-main.c and the helper function it uses. The flatpak_run()
function handles printing usage info for the overall flatpak command,
and subcommands use the usage_error() function to print usage
information, so there's no need for it.
Closes: #2082
Approved by: alexlarsson
7b0e84f527