Don't rely on QtcProcess::errorString()

void RsyncDeployService::createRemoteDirectories()
{
...
    connect(m_mkdir.get(), &QtcProcess::done, this, [this] {
        QString userError;
        const QString error = m_mkdir->errorString();
        if (!error.isEmpty())
                   ^^^
            userError = error;
        if (m_mkdir->exitCode() != 0)
            userError = QString::fromUtf8(m_mkdir->readAllStandardError());
        if (!userError.isEmpty()) {
            emit errorMessage(tr("Failed to create remote directories: %1").arg(userError));
            setFinished();
            return;
        }
        deployFiles();
    });
...
}