Use g_warning() for X errors and X IO errors

Use g_warning() for X errors and X IO errors

Currently fprintf(stderr, ...) is used for X error and X IO errors
(unless compiled with debugging, in which case g_error() is used for
X errors.)

But if an application is redirecting log messages, we really want
X errors to appear in that same log, so it's better to use a g_logv()
variant.

Though g_warning() isn't really appropriate for "lost connection to the
server", G_LOG_LEVEL_INFO isn't part of the "prefixed log levels"
so will produce output without the application name and PID.

https://bugzilla.gnome.org/show_bug.cgi?id=630216