Error when gdb_is_target_1 is called without running gdb instance

Error when gdb_is_target_1 is called without running gdb instance

This is a snafu that I encountered while implementing the previous
patch, which attempted to use gdb_is_target_native.  This proc and
gdb_is_target_remote both rely on gdb_is_target_1, which actually
cannot be called without gdb already running.

This patch adds appropriate warning comments to these procs and
causes gdb_is_target_1 to issue a Tcl error if it is called without a
gdb instance already running.  This should prevent unwitting callers
from using this at the wrong time.