Add more context aware error for policy parsing errors (#8726)

In existing functionality we simply return a generic
error such as "MalformedPolicy" which indicates just
a generic string "invalid resource" which is not very
meaningful when there might be multiple types of errors
during policy parsing. This PR ensures that we send
these errors back to client to indicate the actual
error, brings in two concrete types such as

 - iampolicy.Error
 - policy.Error

Refer #8202

cmd/api-errors.go

@@ -34,6 +34,7 @@ import (
 	"github.com/minio/minio/pkg/auth"
 	"github.com/minio/minio/pkg/event"
 	"github.com/minio/minio/pkg/hash"
+	"github.com/minio/minio/pkg/policy"
 )
 
 // APIError structure
@@ -1772,6 +1773,12 @@ func toAPIError(ctx context.Context, err error) APIError {
 		// their internal error types. This code is only
 		// useful with gateway implementations.
 		switch e := err.(type) {
+		case policy.Error:
+			apiErr = APIError{
+				Code:           "MalformedPolicy",
+				Description:    e.Error(),
+				HTTPStatusCode: http.StatusBadRequest,
+			}
 		case crypto.Error:
 			apiErr = APIError{
 				Code:           "XKMSInternalError",