compositor: print a clear message when XDG_RUNTIME_DIR is not set
compositor: print a clear message when XDG_RUNTIME_DIR is not set

Without this patch, Weston would not start with the message:
creating a keymap file for 44012 bytes failed: No such file or directory

If you do not know, that we use XDG_RUNTIME_DIR for that, the message is
very confusing. Therefore implement a clear error message right at the
start if XDG_RUNTIME_DIR is not set.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

Tarnyko/weston-xdg_surface_present,Gnurou/weston,mchalupa/weston,jonnylamb/weston,krezovic/weston,giucam/weston,Tarnyko/weston-xdg_surface_present,kwm81/weston,jonnylamb/weston,Gnurou/weston,sir-murray/weston,sir-murray/weston,Fantu/compositor-spice,mchalupa/weston,kwm81/weston,eyolfson/weston,giucam/weston,udoprog/weston,udoprog/weston,eyolfson/weston,xorgy/weston,xorgy/weston,krezovic/weston,Fantu/compositor-spice,Fantu/compositor-spice