import IntegrityError used as base class with according name

This not only brings code style in line with the other helpers that do the
same thing this way, but also does away with an unnecessary absolute import
using the borg module name explicitly.

src/borg/helpers/errors.py

@@ -1,6 +1,6 @@
 from ..constants import *  # NOQA
 
-import borg.crypto.low_level
+from ..crypto.low_level import IntegrityError as IntegrityErrorBase
 
 
 class Error(Exception):
@@ -30,7 +30,7 @@ class ErrorWithTraceback(Error):
     traceback = True
 
 
-class IntegrityError(ErrorWithTraceback, borg.crypto.low_level.IntegrityError):
+class IntegrityError(ErrorWithTraceback, IntegrityErrorBase):
     """Data integrity error: {}"""