Log what exception caused abort of the transaction

In some cases, when ovsdb transaction is aborted, knowing what error
caused that can help to understand the root cause of the problem
so this patch adds such logging on the DEBUG level.

Change-Id: Ic67575ff26c689ec8ab1a5c619ae0b0a2846b05f

ovsdbapp/backend/ovs_idl/transaction.py

@@ -91,10 +91,17 @@ class Transaction(api.Transaction):
                           {'idx': i, 'cmd': command, 'n': attempts})
                 try:
                     command.run_idl(txn)
-                except Exception:
+                except Exception as e:
                     txn.abort()
                     if self.check_error:
                         raise
+                    if self.log_errors:
+                        LOG.error("txn n=%(n)d command(idx=%(idx)s): %(cmd)s ",
+                                  "aborted due to error: %(err)s",
+                                  {'idx': i,
+                                   'cmd': command,
+                                   'n': attempts,
+                                   'err': e})
             status = txn.commit_block()
             if status == txn.TRY_AGAIN:
                 LOG.debug("OVSDB transaction returned TRY_AGAIN, retrying")