Make subprocess catch mkvmerge exit code 1

[LightArrowsEXE]

mkvmerge returns exit code 1 if a warning is thrown (see: docs). However, currently this forcibly halts the muxing in vardautomation, because it errors before it can actually output anything it appears (and consequently, no cleaning up can be run either).

There should be an additional check to catch the exit code, if possible, and let vardautomation know that it's a non-fatal error and it should otherwise continue (and maybe throw a warning if you want, such as the one stderr should return).