Throws a TypeError when service worker not supported

[gavmck]

Expected Behavior

UpUp.start should fail silently when not supported.

Current Behavior

Throws an error: TypeError: null is not an object (evaluating 'UpUp.start')

Possible Solution

Still return an object from UpUp so you can call UpUp.start without it exploding.

Context

We catch JS errors in our app and forward to /error, which tries to load upup which causes a js error which forwards to /error in an infinite loop

Your Environment

• Version used: 0.3.0
• Browser Name and version: Safari 11.0.2
• Operating System and version (desktop or mobile): OSX