setup wizard cannot handle errors in tarsnap-keygen (or perhaps anything)

[Graham Percival]

When I try to create a new keyfile, I get the "Generating keyfile..." with a "busy" animation for less than a second, then the animation disappears. Looking at the log, I see:

Task {76ec4aeb-e36a-48ae-ba0f-72ff5f8e4e7b} finished with error 1 (Process crashed) occured (exit code 5):
[/usr/local/bin/tarsnap-keygen --user '[e-mail]' --machine Grahams-Mac-mini.local --keyfile '/Users/gperciva/Library/Application Support/Tarsnap Backup Inc./Tarsnap/Grahams-Mac-mini.local-2017-09-01-13-46-18.key']
dyld: lazy symbol binding failed: Symbol not found: _clock_gettime
  Referenced from: /usr/local/bin/tarsnap-keygen
  Expected in: /usr/lib/libSystem.B.dylib

dyld: Symbol not found: _clock_gettime
  Referenced from: /usr/local/bin/tarsnap-keygen
  Expected in: /usr/lib/libSystem.B.dylib

Fair enough; that's a known problem (Tarsnap/tarsnap#286) and I can fix that with a brew uninstall tarsnap && brew install tarsnap. However, the GUI should never be in that state!

I see that TarsnapTask::processFinished() will only emit finished(...) if we get _process->exitStatus() == Process::NormalExit. As a result,

void TaskManager::registerMachineFinished(QVariant data, int exitCode,
                                          QString stdOut, QString stdErr)
{
    Q_UNUSED(data)
    if(exitCode == SUCCESS)
        emit registerMachineStatus(TaskStatus::Completed, stdOut);
    else
        emit registerMachineStatus(TaskStatus::Failed, stdErr);

will never be called with exitCode != SUCCESS, so of course we won't see the error in the GUI.

I suspect (but haven't yet read all the code) that this applies to all other tasks as well -- errors won't be displayed because it doesn't get a finished signal.