use the latest couchrest_session_store

This one does not use our own error class for connection issues anymore. We'll remove that class in the next commit. So let's not rely on it.
reraise with a better explaination on couch failure

Removing our own error class for this. It interferes with couch_rest_session_store tryign to catch the same errors.
minor: rename test
proceed even if the couch is unreachable

* in case the user has a session id, keep it but proceed without a session
* in case we can't initialize the models proceed
* if APP_CONFIG[:reraise_errors] is set we'll crash instead in the latter case

default to reraise errors in dev and test environments.
Merge pull request #121 from azul/feature/robust-against-couch-connection-issues

Feature/robust against couch connection issues