avatar: Remove unnecessary try...except statement.
avatar: Remove unnecessary try...except statement.

The `RateLimited` exception can be caught by `JsonErrorHandler`, so it
is not necessary to have the try...except statement here. It is also invalid
to pass a string to initialize `RateLimited`.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip