Missing file return value should be 2 like in Black

[akaihola]

If a file given on the command line doesn't exist, Darker exits with a return value of 1. This is the same return value as returned from darker --check if the file needs reformatting.

It would be better to match Black's return value of 2 when a file doesn't exist.

Darker 2.1.1's behavior:

$ darker /tmp/foobar.py ; echo $?
argument PATH: Error: Path(s) '/tmp/foobar.py' do not exist in the working tree
1

Black's behavior:

$ black /tmp/foobar.py ; echo $?
Usage: black [OPTIONS] SRC ...
Try 'black -h' for help.

Error: Invalid value for 'SRC ...': Path '/tmp/foobar.py' does not exist.
2