Handle invald or misconfigured worker address collection

[KPrasch]

Feedback can be supplied during worker address collection:

• invalid checksum address
• connected wattet is the same as worker
• address is already used as a staker
• eth balance too / short runway

Example from the spec:

[cygnusv]

This is what I see now.

[cygnusv]

Also, there's no way to going back to correct the address (except by pressing the back button in the browser, which takes you basically at the starting point)

[derekpierre]

This is what I see now.

@cygnusv the broken 'x' image should be fixed via #84 , and that allows you to re-enter a new worker address. You can actually click the broken image and the functionality works.