Catch expired token or bad token error and print friendlier message

Currently throws a stack trace:

```
# sd list -r
/Users/bweaver/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/excon-0.32.0/lib/excon/middlewares/expects.rb:6:in `response_call': Expected(200) <=> Actual(403 Forbidden) (Excon::Errors::Forbidden)
response => #"\nExpiredTokenThe security token included in the request is expired50d0fc32-d2e1-479e-8926-d52a0869ea8a", :headers=>{"Transfer-Encoding"=>"", "Date"=>"Wed, 20 Aug 2014 19:55:56 GMT", "Server"=>"Amazon SimpleDB"}, :status=>403, :remote_ip=>"205.251.235.213", :local_port=>59347, :local_address=>"172.28.164.21"}, @body="\nExpiredTokenThe security token included in the request is expired50d0fc32-d2e1-479e-8926-d52a0869ea8a", @headers={"Transfer-Encoding"=>"", "Date"=>"Wed, 20 Aug 2014 19:55:56 GMT", "Server"=>"Amazon SimpleDB"}, @status=403, @remote_ip="205.251.235.213", @local_port=59347, @local_address="172.28.164.21">
```