gverilla

Explicitly handle error, when .install isn't executable

2

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](https://github.com/EvanPurkhiser/dots/blob/d25236998aa609d1aa0586ffe2d5208747cc318b/bin/dots#L506), when...