Compress Merge -> check if zip file is writable before doing anything

[MattMills]

Is your feature request related to a problem? Please describe.
I often re-compress merge a modset without realizing I still have one of the zip files open in 7zip, causing Irony to crash.

Describe the solution you'd like
It'd obviously be better if it didn't crash, but said the file wasn't accessible/writable and to check if it's open in another program.

Describe alternatives you've considered
I've considering taping a sticky note to my forehead telling me to close mod zip files before trying to overwrite them, but it got in the way of day to day activities.

Additional context
N/A