Display better error when hooks already exist

When non-Overcommit hooks were already installed in a repo, Overcommit
would raise an exception. Change the behavior to gracefully catch the
exception and display a normal error message.

Change-Id: Ibc11c3d9f476a47c403133ebe1b90f138078a1ca
Reviewed-on: http://gerrit.causes.com/36003
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>

Author: Shane da Silva

CHANGELOG.md

@@ -6,6 +6,8 @@
   instead of a shell string
 * Rename `command` hook helper to `execute`
 * Add support for JRuby 1.7.9 in Ruby 1.9 mode
+* Display more helpful error message when installing Overcommit into a repo
+  that already has non-Overcommit hooks
 
 ## 0.6.3
 

lib/overcommit/cli.rb

@@ -81,6 +81,9 @@ def install_or_uninstall
         rescue Overcommit::Exceptions::InvalidGitRepo => error
           log.warning "Invalid repo #{target}: #{error}"
           halt 69 # EX_UNAVAILABLE
+        rescue Overcommit::Exceptions::PreExistingHooks => error
+          log.warning "Unable to install into #{target}: #{error}"
+          halt 73 # EX_CANTCREAT
         end
       end
     end