Display better error when unable to install

When attempting to install Overcommit as part of running the hook, we
didn't give an informative error if for whatever reason this install
failed.

Improve the error message and change the exit status to follow the
`sysexits` standard.

Change-Id: I91fe79932a93cc9df5a7568fd223287c956e0f27
Reviewed-on: http://gerrit.causes.com/35620
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>

Author: sds

template-dir/hooks/overcommit-hook

@@ -30,8 +30,8 @@ rescue LoadError
       Gem.clear_paths # Reset load paths so newly installed gem is found
       require 'overcommit'
     else
-      puts 'Unable to install Overcommit'
-      exit
+      puts 'Unable to install Overcommit -- try running `gem install overcommit`'
+      exit 69 # EX_UNAVAILABLE
     end
   else
     puts 'You chose not to install Overcommit'