Catch http-conduit failures and wrap them up as errors that are visible in api

As it currently exists, it is really easy to get ResponseTimeouts, which are completely invisible in the api provided by hs-stripe. I did two things to fix this:

1. Make the default timeout longer. Before doing this, I would easily get 5-10% of requests timing out, from a east coast datacenter with a fast connection. After the change, it dropped down to <1%. 
2. Actually catch HttpExceptions that are thrown by http-conduit, and create a new HttpFailure variant of StripeFailure, and pass them back.