Rename func type suffixes Fn->Func and add OnUsageErrorFunc

funcs.go

@@ -1,18 +1,24 @@
 package cli
 
 // An action to execute when the bash-completion flag is set
-type BashCompleteFn func(*Context)
+type BashCompleteFunc func(*Context)
 
 // An action to execute before any subcommands are run, but after the context is ready
 // If a non-nil error is returned, no subcommands are run
-type BeforeFn func(*Context) (int, error)
+type BeforeFunc func(*Context) (int, error)
 
 // An action to execute after any subcommands are run, but after the subcommand has finished
 // It is run even if Action() panics
-type AfterFn func(*Context) (int, error)
+type AfterFunc func(*Context) (int, error)
 
 // The action to execute when no subcommands are specified
-type ActionFn func(*Context) int
+type ActionFunc func(*Context) int
 
 // Execute this function if the proper command cannot be found
-type CommandNotFoundFn func(*Context, string)
+type CommandNotFoundFunc func(*Context, string)
+
+// Execute this function if an usage error occurs. This is useful for displaying
+// customized usage error messages.  This function is able to replace the
+// original error messages.  If this function is not set, the "Incorrect usage"
+// is displayed and the execution is interrupted.
+type OnUsageErrorFunc func(context *Context, err error, isSubcommand bool) error