implement handler for `QSetDetachOnError` packet

[andrurogerz]

ds2 does not implement a handler for the QSetDetachOnError packet defined here.

The following warning is observed in the ds2 output when running lldb tests:

[9847][void ds2::GDBRemote::ProtocolInterpreter::onCommand(const std::string &, const std::string &)] WARNING: handler for command 'QSetDetachOnError' unknown