Add helpful error message for gem loading issues in hooks

Previously, if you had a `require` statement in a hook for a gem that
wasn't installed or wasn't specified in the loaded Gemfile, you would
get an unhelpful load error.

Provide some helpful guidance in this case since it's almost always the
problem when it comes to load errors, and people will likely forget to
add a gem to their custom `.overcommit_gems.rb` file if they have one.

Author: sds

lib/overcommit/configuration.rb

@@ -18,6 +18,14 @@ def ==(other)
     end
     alias_method :eql?, :==
 
+    # Access the configuration as if it were a hash.
+    #
+    # @param key [String]
+    # @return [Array,Hash,Number,String]
+    def [](key)
+      @hash[key]
+    end
+
     # Returns absolute path to the directory that external hook plugins should
     # be loaded from.
     def plugin_directory

lib/overcommit/hook_runner.rb

@@ -144,6 +144,18 @@ def load_hooks
 
       # Load plugin hooks after so they can subclass existing hooks
       @hooks += HookLoader::PluginHookLoader.new(@config, @context, @log).load_hooks
+    rescue LoadError => ex
+      # Include a more helpful message that will probably save some confusion
+      message = 'A load error occurred. ' +
+        if @config['gemfile']
+          "Did you forget to specify a gem in your `#{@config['gemfile']}`?"
+        else
+          'Did you forget to install a gem?'
+        end
+
+      raise Overcommit::Exceptions::HookLoadError,
+            "#{message}\n#{ex.message}",
+            ex.backtrace
     end
   end
 end

template-dir/hooks/overcommit-hook

@@ -75,6 +75,10 @@ rescue Overcommit::Exceptions::HookContextLoadError => error
   puts error
   puts 'Are you running an old version of Overcommit?'
   exit 69 # EX_UNAVAILABLE
+rescue Overcommit::Exceptions::HookLoadError => error
+  puts error.message
+  puts error.backtrace
+  exit 78 # EX_CONFIG
 rescue Overcommit::Exceptions::HookSetupFailed,
        Overcommit::Exceptions::HookCleanupFailed => error
   puts error.message