Move error-handling routines & co to lab.nexedi.com/kirr/go123

error.go is completely being moved to that shared place for handy Go
utilities into several subpackages:

	lab.nexedi.com/kirr/go123/exc		-- exception-style error handling for Go
	lab.nexedi.com/kirr/go123/myname	-- easy way to determine current function's name and package
	lab.nexedi.com/kirr/go123/xerr		-- addons for error-handling
	lab.nexedi.com/kirr/go123/xruntime	-- addons to standard package runtime