Description of problem:
 
When user double clicks unsubscribe/subscribe buttons in a system's subscriptions view, errors are thrown. We should capture and handle these gracefully, and possible just disable multiple click of the buttons

Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1.  Sync and promote content
2.  Register system
3.  Systems > $system > Subscriptions
4.  Select one or more items in Available Subscriptions and "accidentally" hit 'Subscribe' button twice
5.  Similarly, select item(s) from System Current Subscription(s) and again "accidentally" hit Unsubscribe button
Actual results:

Candlepin::Consumer: 403 Forbidden {"displayMessage":"This consumer is already subscribed to the product matching pool with id 'ff808081359b2e8b01359c7fcc7a006b'."} (POST /candlepin/consumers/e2ab8cd9-6bc7-41e7-b740-acb3c05c37f4/entitlements?pool=ff808081359b2e8b01359c7fcc7a006b&quantity=1)

Candlepin::Consumer: 404 Resource Not Found {"displayMessage":"Entitlement with ID 'ff808081359b2e8b0135a3305a8e0089' could not be found."} (DELETE /candlepin/consumers/e2ab8cd9-6bc7-41e7-b740-acb3c05c37f4/entitlements/ff808081359b2e8b0135a3305a8e0089): {"displayMessage":"Entitlement with ID 'ff808081359b2e8b0135a3305a8e0089' could not be found."}

Expected results:
Handle these errors cleanly
and/or
Disable multiple clicks of these buttons

Additional info:

Interestingly, these errors do not make their way into the notifications view.