Improve error message for missing gemfile dependencies

If you hadn't run `bundle install --gemfile=<overcommit-gemfile>`, you
would get an error during a hook run that lead you to believe there was
a problem with your `Gemfile`, when it was the separate Overcommit gem
file.

Fix this by improving the error message to make this clearer.

Author: sds

CHANGELOG.md

@@ -7,6 +7,7 @@
   [`mdl`](https://github.com/mivok/markdownlint) on Markdown files
 * Add `--without-color` flag to `RailsBestPractices` pre-commit hook
   to fix parsing issues due to color escape sequences
+* Improve error message when `gemfile` has not had a dependency installed
 
 ## 0.30.0
 

template-dir/hooks/overcommit-hook

@@ -30,7 +30,15 @@ require 'yaml'
 # rubocop:disable Style/RescueModifier
 if gemfile = YAML.load_file('.overcommit.yml')['gemfile'] rescue nil
   ENV['BUNDLE_GEMFILE'] = gemfile
-  require 'bundler/setup'
+  require 'bundler'
+
+  begin
+    Bundler.setup
+  rescue Bundler::BundlerError => ex
+    puts "Problem loading '#{gemfile}': #{ex.message}"
+    puts "Try running:\nbundle install --gemfile=#{gemfile}" if ex.is_a?(Bundler::GemNotFound)
+    exit 78 # EX_CONFIG
+  end
 end
 # rubocop:enable Style/RescueModifier