Comment 0 for bug 1583624

From mir-greeter.log (and with minor instrumentation from RAOF)

ERROR: QMirServer - Mir failed to start
qtmir.screens: ScreenController::ScreenController
qtmir.mir: MirServer created
qtmir.mir: Command line arguments passed to Qt: ("/usr/bin/unity8", "--mode=greeter")
ERROR: /home/josh/Documents/mir/verbose-error-for-vt-finding/src/server/graphics/default_configuration.cpp(133): Throw in function mir::DefaultServerConfiguration::the_graphics_platform()::<lambda()>
Dynamic exception type: boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<std::runtime_error> >
std::exception::what: Exception while creating graphics platform
ERROR: /home/josh/Documents/mir/verbose-error-for-vt-finding/src/platforms/mesa/server/kms/linux_virtual_terminal.cpp(238): Throw in function int mir::graphics::mesa::LinuxVirtualTerminal::find_active_vt_number()
Dynamic exception type: boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<std::runtime_error> >
std::exception::what: Failed to find the current VT. Tried:
 /dev/tty0:
  Failed to open read-only: Permission denied
  Failed to open write-only: Permission denied
 /dev/tty:
  Failed to open read-only: No such device or address
  Failed to open write-only: No such device or address

This occurs when trying to start the unity8 greeter under unity system compositor