Crash on invalid log file path

[szszszsz]

• Application version: v1.3.2 - current
• Operating system: macOS 10.13/10.14
• Device model and firmware version: Pro/Storage
• Issue occurrence: always

Expected behavior

App should not crash. On an invalid log path set, message should be shown to user, and the log should not start.

Current behavior

App crashes, when set log file path is not correct (no write access?)

Steps for reproduction

1. Set invalid path for debug log
2. Close and start App again

Logs

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   org.qt-project.QtCore         	0x000000010d6436fd QFileDevice::flush() + 13
1   com.nitrokey.nitrokey-app     	0x000000010c774eb6 0x10c2c5000 + 4914870
2   com.nitrokey.nitrokey-app     	0x000000010c77475c 0x10c2c5000 + 4912988
3   com.nitrokey.nitrokey-app     	0x000000010c774419 0x10c2c5000 + 4912153
4   com.nitrokey.nitrokey-app     	0x000000010c2da7ce std::__1::function<void (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >)>::operator()(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) const + 142
5   com.nitrokey.nitrokey-app     	0x000000010c2da6eb nitrokey::log::FunctionalLogHandler::print(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, nitrokey::log::Loglevel) + 91
6   com.nitrokey.nitrokey-app     	0x000000010c2d8a3c nitrokey::log::Log::operator()(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, nitrokey::log::Loglevel) + 140
7   com.nitrokey.nitrokey-app     	0x000000010c2d2585 nitrokey::device::Device::could_be_enumerated() + 261
8   com.nitrokey.nitrokey-app     	0x000000010c2e9035 nitrokey::NitrokeyManager::is_connected() + 165