Run auto-spell-checker (codespell) on files in depot_tools.

This should only change spelling in:
 - messages that are printed
 - comments
 - docstrings

Review-Url: https://codereview.chromium.org/2277513002
diff --git a/gerrit_util.py b/gerrit_util.py
index 66b205f..77fcb07 100755
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -326,7 +326,7 @@
     # A status >=500 is assumed to be a possible transient error; retry.
     http_version = 'HTTP/%s' % ('1.1' if response.version == 11 else '1.0')
     msg = (
-        'A transient error occured while querying %s:\n'
+        'A transient error occurred while querying %s:\n'
         '%s %s %s\n'
         '%s %d %s' % (
             conn.host, conn.req_params['method'], conn.req_params['url'],