Fixed an inverted check for the mask list.
Fixed an inverted check for the mask list.

When using a mask list in the track_features tool, we were throwing
an error if the file exists instead of when it doesn't exist.

linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk