Fix copy of vim directory

The variable $sachet__install_home already contains a trailing slash, no need to add one when concatening the directory/file to it
Activate exit on error

I think it's a good idea to exit on error as any error means we could not backup previous configuration or did not entirely install the modules or the vimrc file hence we should not echo that sachet had successfully been installed
Add ashemsay for install.sh fix
Merge pull request #28 from ashemsay/master

Fix install.sh