verify we can access the display early

[Antoine Martin]

Bail out of the client / shadow server if we can't.

[Antoine Martin]

Done in r25884 + r25885.

[stdedos]

Is that for MacOS only?

I think I've seen a case that I cannot start the shadow server for a Bionic laptop, freshly rebooted.

u@h:~$ xpra shadow ssh://ho@ho
2020-03-31 16:45:45,066 Xpra GTK3 X11 client version 3.0.8-r25767 64-bit
2020-03-31 16:45:45,130  running on Linux Ubuntu 16.04 xenial
2020-03-31 16:45:45,131  window manager is 'Xpra'
2020-03-31 16:45:45,278 No OpenGL_accelerate module loaded: No module named 'OpenGL_accelerate'

(xpra:2406): Gdk-ERROR **: The program 'xpra' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadValue (integer parameter out of range for operation)'.
  (Details: serial 231 error_code 2 request_code 151 (GLX) minor_code 3)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
Trace/breakpoint trap (core dumped)
u@h:~$ ssh ho@ho
# Bionic here
ho@ho:~$ xpra shadow :0
ho@ho:~$ Entering daemon mode; any further errors will be reported to:
  /run/user/1000/xpra/display-:0-20200331-164817.log
cd /run/user/1000/xpra/
ho@ho:~$ cat display-\:0-20200331-164817.log 
2020-03-31 16:48:20,360 Error: failed to connect to display :0
2020-03-31 16:48:20,360  could not connect to X server on display ':0' after 3 seconds