Explicitly handle error, when .install isn't executable

[gverilla]

Currently, when an .install-script that isn't executable is to be executed, the application fails with a cryptic:

[Errno 13]: Permission denied

(which is what subprocess.Popen throws here, when it can't run a script) That's a terrible error message to debug.

Instead I'd suggest to handle it either by a) attempting to chmod +x it manually or by b) exiting with a message that explains what script caused the error.

[gverilla]

I just send you a PR for the "exiting with an explanation"-scenario.

[countsudoku]

+1