Revert UsageOnMissing.

This was breaking UsageOnMissing() when combined with validation of
positional arguments, in that it was allowing positional arguments to be
omitted. It also broke UsageOnMissing() somehow so that it did not work.

context.go

@@ -683,14 +683,8 @@ func (c *Context) Run(binds ...interface{}) (err error) {
 	defer catch(&err)
 	node := c.Selected()
 	if node == nil {
-		if c.Kong.usageOnMissing {
-			return c.PrintUsage(false)
-		}
 		return fmt.Errorf("no command selected")
 	}
-	if c.Kong.usageOnMissing && isMissingChildError(c.Error) {
-		return c.PrintUsage(false)
-	}
 	return c.RunNode(node, binds...)
 }
 
@@ -759,7 +753,13 @@ func checkMissingChildren(node *Node) error {
 		return nil
 	}
 
-	return newMissingChildError(missing)
+	if len(missing) > 5 {
+		missing = append(missing[:5], "...")
+	}
+	if len(missing) == 1 {
+		return fmt.Errorf("expected %s", missing[0])
+	}
+	return fmt.Errorf("expected one of %s", strings.Join(missing, ",  "))
 }
 
 // If we're missing any positionals and they're required, return an error.