diff --git a/error.py b/error.py
index 5238158..812585c 100644
--- a/error.py
+++ b/error.py
@@ -57,6 +57,15 @@
   def __str__(self):
     return self.reason
 
+class DownloadError(Exception):
+  """Cannot download a repository.
+  """
+  def __init__(self, reason):
+    self.reason = reason
+
+  def __str__(self):
+    return self.reason
+
 class NoSuchProjectError(Exception):
   """A specified project does not exist in the work tree.
   """