Guarantee that _dostore() does not leave a transaction in progress.

Put the tpc_begin() -> tpc_finish() in a try/except.  If an error
occurs, abort the current transaction and re-raise the error.

It appears that some storages (well, only Standby Storage) has trouble
shutting down if a transaction is in progress.

src/ZODB/tests/StorageTestBase.py

@@ -153,6 +153,7 @@ class StorageTestBase(unittest.TestCase):
             t.user = user
         if description is not None:
             t.description = description
+        try:
             self._storage.tpc_begin(t)
             # Store an object
             r1 = self._storage.store(oid, revid, data, version, t)
@@ -160,6 +161,9 @@ class StorageTestBase(unittest.TestCase):
             r2 = self._storage.tpc_vote(t)
             revid = handle_serials(oid, r1, r2)
             self._storage.tpc_finish(t)
+        except:
+            self._storage.tpc_abort(t)
+            raise
         return revid
         
     def _dostoreNP(self, oid=None, revid=None, data=None, version=None,