cmd: use regoError to carry compiler errors into CLI machinery

Signed-off-by: Stephan Renatus <stephan@styra.com>

cmd/eval.go

@@ -175,12 +175,18 @@ const (
 	defaultPrettyLimit  = 80
 )
 
-type regoError struct{}
+type regoError struct {
+	wrapped error
+}
 
 func (regoError) Error() string {
 	return "rego"
 }
 
+func (r regoError) Unwrap() error {
+	return r.wrapped
+}
+
 func init() {
 	params := newEvalCommandParams()
 
@@ -304,7 +310,7 @@ access.
 				if _, ok := err.(regoError); !ok {
 					fmt.Fprintln(os.Stderr, err)
 				}
-				return newExitError(2)
+				return newExitErrorWrap(2, err)
 			}
 
 			if (params.fail && !defined) || (params.failDefined && defined) {
@@ -448,7 +454,7 @@ func eval(args []string, params evalCommandParams, w io.Writer) (bool, error) {
 		// If the rego package returned an error, return a special error here so
 		// that the command doesn't print the same error twice. The error will
 		// have been printed above by the presentation package.
-		return false, regoError{}
+		return false, regoError{wrapped: result.Errors}
 	} else if len(result.Result) == 0 {
 		return false, nil
 	}

cmd/utils.go

@@ -1,4 +1,4 @@
-// Copyright 2023 The OPA Authors.  All rights reserved.
+// Copyright 2025 The OPA Authors.  All rights reserved.
 // Use of this source code is governed by an Apache2
 // license that can be found in the LICENSE file.
 
@@ -9,13 +9,22 @@ import (
 )
 
 type ExitError struct {
-	Exit int
+	Exit    int
+	wrapped error
 }
 
 func newExitError(exit int) error {
 	return &ExitError{Exit: exit}
 }
 
+func newExitErrorWrap(exit int, err error) error {
+	return &ExitError{Exit: exit, wrapped: err}
+}
+
 func (c *ExitError) Error() string {
 	return fmt.Sprintf("exit %d", c.Exit)
 }
+
+func (c *ExitError) Unwrap() error {
+	return c.wrapped
+}