refactor: ParseError can carry an exit code

Reinstated use of ParseError for all error paths so as to retain existing semantics.

error.go

@@ -5,11 +5,17 @@ package kong
 // It contains the parse Context that triggered the error.
 type ParseError struct {
 	error
-	Context *Context
+	Context  *Context
+	exitCode int
 }
 
 // Unwrap returns the original cause of the error.
 func (p *ParseError) Unwrap() error { return p.error }
 
 // ExitCode returns the status that Kong should exit with if it fails with a ParseError.
-func (p *ParseError) ExitCode() int { return exitUsageError }
+func (p *ParseError) ExitCode() int {
+	if p.exitCode == 0 {
+		return exitNotOk
+	}
+	return p.exitCode
+}