Merged r17960 to 4.0-stable (#31030).

git-svn-id: http://svn.redmine.org/redmine/branches/4.0-stable@17961 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine/plugin.rb

@@ -269,7 +269,11 @@ module Redmine
       arg = { :version_or_higher => arg } unless arg.is_a?(Hash)
       arg.assert_valid_keys(:version, :version_or_higher)
 
-      plugin = Plugin.find(plugin_name)
+      begin
+        plugin = Plugin.find(plugin_name)
+      rescue PluginNotFound
+        raise PluginRequirementError.new("#{id} plugin requires the #{plugin_name} plugin")
+      end
       current = plugin.version.split('.').collect(&:to_i)
 
       arg.each do |k, v|