cli: enhance error messages when Kubernetes is not running

[diegodelemos]

Stemmed from reanahub/reana-client#304

When Minikube is not running or the REANA pods are not working, reana-dev setup-environment errors out with cryptic errors:

$ # Minikube not running example
$ reana-dev setup-environment
Environment variables could not be generated: 
HTTPSConnectionPool(host='192.168.39.111', port=8443): Max retries exceeded with url: /api/v1/nodes (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7f728e908f10>: Failed to establish a new connection: [Errno 113] No route to host',))
$ # DB pod down or inaccessible
$ reana-dev setup-environment
Environment variables could not be generated: 
Command '['kubectl', 'exec', '-t', u'db-54c5c6cb84-kvwph', '--', 'psql', '-U', 'reana', 'reana', '-c', 'SELECT access_token FROM user_']' returned non-zero exit status 1