Add a global network error handler, hooked inside of Bluebird (which is a stupid thing to do, but since we're only using Bluebird for network operations as of now, it kinda works).

See #16
Downgrade React to v0.10
Handle both global and uncaught Promise errors.

<ErrorHandler /> now shows them one after the other if the user choose to dismiss the dialog instead of reloading the page.

See #16
Show different modals for network and generic errors.

Closes #16