Use "Unresolved reference" error message when no symbols were found

	in a symbolic reference resolution. Remove .expr and -expr from
	the shown reference when the reference is unresolved.

	* src/scan-code.l: Change the error message, adjust location columns,
	rename variable "exact_mode" to "explicit_bracketing".
	* tests/named-ref.at: Adjust existing tests and add a new one.