fix transaction_failed

- sqlite3.ErrNo is not an sqlite3.Error, we have to unpack it first
- an interrupted transaction in an in-memory database (with shared
  cache) leads to ErrLocked, while in a regular file-backed databes an
  ErrBusy is returned

goatherd.go

@@ -164,8 +164,10 @@ func get_otp(tx *sql.Tx, name string) (*hotp.HOTP, error) {
 }
 
 func transaction_failed(err error) bool {
-    // XXX: check type first?
-    return err == sqlite3.ErrBusy
+    if err, ok := err.(sqlite3.Error); ok {
+        return err.Code == sqlite3.ErrLocked || err.Code == sqlite3.ErrBusy
+    }
+    return false
 }
 
 // Retrieve secret and count for given username and try to find a match within