Export rpmteFailed() to python bindings

Export rpmteFailed() to python bindings

- Kinda related to RhBug:661962, yum relies on callbacks to catch
  install/erase errors but this is not accurate: on erase the
  element can be ambiguous as the callback only gives a name (sigh).
  In addition, elements can be skipped entirely if "parent" element
  fails, in which case no callbacks are issued so these cases would go
  completely unnoticed when relying on callbacks alone. te.Failed()
  gives users such as yum a chance to have a look at the real status
  of elements (after the transaction).