Do not fail if bt5 file can't be read

product/ERP5/Document/BusinessTemplate.py

@@ -5028,7 +5028,7 @@ Business Template is a set of definitions, such as skins, portal types and categ
         from Products.ERP5VCS.WorkingCopy import NotAWorkingCopyError
         try:
           self.setRevision(self.getVcsTool().newRevision())
-        except NotAWorkingCopyError:
+        except (NotAWorkingCopyError, IOError):
           raise ImportError
       except ImportError:
         self.updateRevisionNumber()