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
+}

kong.go

@@ -312,34 +312,34 @@ func (k *Kong) extraFlags() []*Flag {
 func (k *Kong) Parse(args []string) (ctx *Context, err error) {
 	ctx, err = Trace(k, args)
 	if err != nil { // Trace is not expected to return an err
-		return nil, err
+		return nil, &ParseError{error: err, Context: ctx, exitCode: exitUsageError}
 	}
 	if ctx.Error != nil {
-		return nil, &ParseError{error: ctx.Error, Context: ctx}
+		return nil, &ParseError{error: ctx.Error, Context: ctx, exitCode: exitUsageError}
 	}
 	if err = k.applyHook(ctx, "BeforeReset"); err != nil {
-		return nil, err
+		return nil, &ParseError{error: err, Context: ctx}
 	}
 	if err = ctx.Reset(); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}
 	}
 	if err = k.applyHook(ctx, "BeforeResolve"); err != nil {
-		return nil, err
+		return nil, &ParseError{error: err, Context: ctx}
 	}
 	if err = ctx.Resolve(); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}
 	}
 	if err = k.applyHook(ctx, "BeforeApply"); err != nil {
-		return nil, err
-	}
-	if _, err = ctx.Apply(); err != nil { // Apply is not expected to return an err
-		return nil, err
-	}
-	if err = ctx.Validate(); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}
 	}
+	if _, err = ctx.Apply(); err != nil { // Apply is not expected to return an err
+		return nil, &ParseError{error: err, Context: ctx}
+	}
+	if err = ctx.Validate(); err != nil {
+		return nil, &ParseError{error: err, Context: ctx, exitCode: exitUsageError}
+	}
 	if err = k.applyHook(ctx, "AfterApply"); err != nil {
-		return nil, err
+		return nil, &ParseError{error: err, Context: ctx}
 	}
 	return ctx, nil
 }