Fix zone parser error handling (#6680)

Signed-off-by: Nathan Currier <nathan.currier@broadcom.com>

plugin/file/file_test.go

@@ -29,3 +29,31 @@ www          IN  A      192.168.0.14
 mail         IN  A      192.168.0.15
 imap         IN  CNAME  mail
 `
+
+func TestParseSyntaxError(t *testing.T) {
+	_, err := Parse(strings.NewReader(dbSyntaxError), "example.org.", "stdin", 0)
+	if err == nil {
+		t.Fatalf("Zone %q should have failed to load", "example.org.")
+	}
+	if !strings.Contains(err.Error(), "\"invalid\"") {
+		t.Fatalf("Zone %q should have failed with syntax error: %s", "example.org.", err)
+	}
+}
+
+const dbSyntaxError = `
+$TTL         1M
+$ORIGIN      example.org.
+
+@            IN  SOA    ns1.example.com. admin.example.com.  (
+                               2005011437 ; Serial
+                               1200       ; Refresh
+                               144        ; Retry
+                               1814400    ; Expire
+                               2h )       ; Minimum
+@            IN  NS     ns1.example.com.
+
+# invalid comment
+www          IN  A      192.168.0.14
+mail         IN  A      192.168.0.15
+imap         IN  CNAME  mail
+`