better std::exception logging for CAddrDb

- also small logging text changes
gitian: use right qt tools in linux build

If the `libqt4-dev` package is installed it picks the moc executable
from the system instead of our custom-built one. This results in
compatibility errors.

This commit convinces configure to pick the right one.
Merge pull request #3602

2fdd4c7 better std::exception logging for CAddrDb (Philip Kaufmann)