gui: Translated messages should not be written to debug log (InitError, InitWarning, ...)

Steps to reproduce:

```
$ ./src/qt/bitcoin-qt -lang=nl -blocksdir=/tmp/no/no -printtoconsole | grep 'no/no'
2019-06-15T10:11:27Z Fout: Opgegeven blocks map "/tmp/no/no" bestaat niet.
```

I think to fix this, we'd have to create a function similar to `_(const char* psz)`, that does no translation, but makes `gettext` pick them up and then do something like #15340.