Declare Uh::WM own error classes

lib/uh/wm.rb

@@ -1,3 +1,11 @@
 require 'uh'
 
 require 'uh/wm/cli'
+
+module Uh
+  module WM
+    Error         = Class.new(StandardError)
+    RuntimeError  = Class.new(RuntimeError)
+    ArgumentError = Class.new(Error)
+  end
+end