Turn the error codes that can be returned by both regcomp and regexec into error message strings

Property definitions

core $ NativeRegex :: regerror
	# Turn the error codes that can be returned by both `regcomp` and `regexec` into error message strings
	fun regerror(errcode: Int): CString `{
		size_t len = regerror(errcode, self, NULL, 0);
		char *message = malloc(len);
		regerror(errcode, self, message, len);

		return message;
	`}
lib/core/re.nit:69,2--76,3