Move OtherWMRunningError message in the class

lib/uh/wm.rb

@@ -14,6 +14,11 @@ module Uh
     Error               = Class.new(StandardError)
     RuntimeError        = Class.new(RuntimeError)
     ArgumentError       = Class.new(Error)
-    OtherWMRunningError = Class.new(RuntimeError)
+
+    class OtherWMRunningError < RuntimeError
+      def message
+        'another window manager is already running'
+      end
+    end
   end
 end

lib/uh/wm/manager.rb

@@ -13,9 +13,7 @@ module Uh
       def connect
         @events.emit :connecting, args: @display
         @display.open
-        Display.on_error do
-          fail OtherWMRunningError, 'another window manager is already running'
-        end
+        Display.on_error { fail OtherWMRunningError }
         @display.listen_events INPUT_MASK
         @display.sync false
         Display.on_error { |*args| handle_error *args }