exceptions: Move zerver/exceptions.py under zerver/lib/.

Seems like a more appropriate place for it.  Preparation for
moving a bit more into that file.

zerver/lib/exceptions.py

@@ -0,0 +1,8 @@
+from __future__ import absolute_import
+
+from django.core.exceptions import PermissionDenied
+
+class RateLimited(PermissionDenied):
+    def __init__(self, msg=""):
+        # type: (str) -> None
+        super(RateLimited, self).__init__(msg)