transactionview: make exportClicked() use message()
transactionview: make exportClicked() use message()

- use message() for displaying success or failure of export
- rework the strings to be more detailed / informative
- additional small cleanups

Infernoman/crowncoin,domob1812/crowncoin,Crowndev/crowncoin,Infernoman/crowncoin,syscoin/syscoin,domob1812/crowncoin,syscoin/syscoin,syscoin/syscoin,domob1812/crowncoin,Crowndev/crowncoin,Crowndev/crowncoin,domob1812/crowncoin,domob1812/crowncoin,Crowndev/crowncoin,syscoin/syscoin,domob1812/crowncoin,Infernoman/crowncoin,Infernoman/crowncoin,Crowndev/crowncoin,syscoin/syscoin,Infernoman/crowncoin,syscoin/syscoin,Crowndev/crowncoin,syscoin/syscoin,Infernoman/crowncoin,syscoin/syscoin