Handle `is a directory` error

Closes #4616
Add a better error message when a directory cannot be written to.

Closes #4607
Nof
Specs for the eacces error
:lipstick:
Add initial Windows atom wrapper script
Use $0 instead of %~dp0
Add .sh extension
Install atom.sh shim
Use correct variable name for sh text
Write atom.sh to atom in bin folder
Use forward slashes in sh path
Betta spec description
Add apm sh shim
Include apm.sh
Move up one more directory
Add missing .. segment
Merge pull request #4632 from atom/ks-windows-atom-sh

Add atom.sh and apm.sh Windows scripts
:arrow_up: apm@0.116
Merge pull request #4630 from atom/bo-better-save-notification

Better save notifications