paymentserver: style fixes / cleanup

- rename reportError() into message() to be in line with our default
  message() signal/slot naming (and can be used for all types of messages)
- rename some QStrings to not collide with message() function
- add a missing message for malformed URIs that IS also used in BitcoinGUI
- fix / extend some comments and misc style fixes
Merge pull request #3108 from Diapolo/paymentserver_style

paymentserver: style fixes / cleanup