Check that the owner of a change has name defined

Sometimes Gerrit might not return owner name
due to some internal misconfiguration or database
problem. Since we are using owner name here to
generate the name of a local git branch, instead
of failing here we should try to just go ahead
with 'unknown'.

More information about this error:

https://bugzilla.wikimedia.org/show_bug.cgi?id=40061#c5
https://gerrit.wikimedia.org/r/#/c/37771/

Change-Id: Icaa9baa453e3fd7d54adf9138a9435e4271cee0a

git-review

@@ -712,9 +712,9 @@ def download_review(review, masterbranch, remote):
             topic = review
     except KeyError:
         topic = review
-    if review_info.get('owner'):
+    try:
         author = re.sub('\W+', '_', review_info['owner']['name']).lower()
-    else:
+    except KeyError:
         author = 'unknown'
     branch_name = "review/%s/%s" % (author, topic)