Fix up one-click registration interaction

Fix two issues: 
* #1727 - connection errors were not caught and reported in `doRequest` and `handleFailedAPI`.  Catch those errors and report them to the user.
* #1740 part A - colors for one-click registration were no good (light green, grey on hover); changed to light-green / dark green on hover.
* #1740 part B - on error, disabled button wasn't being re-enabled.  So, re-enable button if error in one-click registration.

**Testing:**
* Load registration page
* Break connection to central server
* Click the `one-click registration` button; should show an error message.