Re: [Libguestfs] [PATCH] handle: Improve error messaging if XDG_RUNTIME_DIR path does not exist.

If an environment variable such as XDG_RUNTIME_DIR or one of the tmpdirs or cachedir is set to a non-existent directory, improve the error message that the user will see so that (where possible) it includes the environment variable or API call. This is still not bullet-proof because it's hard to display the environment variable if it is LIBGUESTFS_TMPDIR or LIBGUESTFS_CACHEDIR, but the main problem is with XDG_RUNTIME_DIR (because of systemd bugs). Thanks: Hilko Bengen for identifying the bug. ---