Don't derive BackReferenceError from FSError. See comments on r773.

fs/errors.py

@@ -188,9 +188,8 @@ class NoMMapError(ResourceError):
     default_message = "Can't get mmap for %(path)s"
 
 
-class BackReferenceError(FSError, ValueError):
+class BackReferenceError(ValueError):
     """Exception raised when too many backrefs exist in a path (ex: '/..', '/docs/../..')."""
-    default_message = "Too many backrefs in '%(path)s'"
 
 
 def convert_fs_errors(func):

fs/path.py

@@ -60,7 +60,7 @@ def normpath(path):
         # Imported here because errors imports this module (path),
         # causing a circular import.
         from fs.errors import BackReferenceError
-        BackReferenceError(details={ 'path': path })
+        BackReferenceError('Too many backrefs in \'%s\'' % path)
     if path[0] == '/':
         return '/%s' % '/'.join(components)
     return '/'.join(components)