Use a better error message when trying to merge non-objects (#6805)

Fixes #6803

Signed-off-by: Anders Eknert <anders@styra.com>

loader/errors.go

@@ -41,7 +41,7 @@ func (e *Errors) add(err error) {
 type unsupportedDocumentType string
 
 func (path unsupportedDocumentType) Error() string {
-	return string(path) + ": bad document type"
+	return string(path) + ": document must be of type object"
 }
 
 type unrecognizedFile string

loader/loader_test.go

@@ -732,7 +732,7 @@ func TestLoadErrors(t *testing.T) {
 		}
 
 		expected := []string{
-			"bad_doc.json: bad document type",
+			"bad_doc.json: document must be of type object",
 			"a.json: EOF",
 			"b.yaml: error converting YAML to JSON",
 			"empty.rego:0: rego_parse_error: empty module",