Remove `ImportError` class

The definition of `ImportError` redefines the Python built-in
class of the same name.

It is not used anywhere, so remove it.

Change-Id: I557ce28c93a3306fff72873dc6f477330fc33128

error.py

@@ -39,15 +39,6 @@ class GitError(Exception):
   def __str__(self):
     return self.command
 
-class ImportError(Exception):
-  """An import from a non-Git format cannot be performed.
-  """
-  def __init__(self, reason):
-    self.reason = reason
-
-  def __str__(self):
-    return self.reason
-
 class UploadError(Exception):
   """A bundle upload to Gerrit did not succeed.
   """