Allow custom fail messages in patchscripts

[JeodC]

The default is still "Patching failed! Please go to the PortMaster Discord for help." however with this addition, patchscripts can override with their own failure messages. This is more verbose for the end user and more helpful for the ecosystem with things outside of PortMaster that make use of the patcher gui, and also helpful in not directing everything that uses it to the PortMaster Discord.

In practice I tested with the following:

emit_fail_msg() {
    echo "PATCH_FAIL_MSG:$1" >&3
    echo "$1"
}

if [ -z "$VCDIFF" ] || [ ! -x "$VCDIFF" ]; then
    emit_fail_msg "Patching failed: vcdiff tool missing. Try reinstalling the port."
    return 1
fi

This in turn shows both the error to the gui dialog and provides a clean log text file:

GAMEDIR is set to: /roms/ports/zelda-ladxhd/data
Preparing system...
Patching failed: vcdiff tool missing. Try reinstalling the port.
Patching process failed!

Thank you for your efforts and for keeping your tools available for all.

[JanTrueno]

Nice addition.