Changes message string

Also DRY by putting it into a constant. Closes Raku/old-issue-tracker#6588 when accepted.

src/core.c/IO/Path.pm6

@@ -6,13 +6,14 @@ my class IO::Path is Cool does IO {
     has $!os-path;        # the absolute path associated with path/SPEC/CWD
     has $!parts;          # IO::Path::Parts object, if any
 
+    constant empty-path-message = "Must specify a non-empty string as a path";
+
     multi method ACCEPTS(IO::Path:D: Cool:D \other) {
         nqp::hllbool(nqp::iseq_s($.absolute, nqp::unbox_s(other.IO.absolute)));
     }
 
     method !SET-SELF(Str:D \path, IO::Spec \SPEC, Str:D \CWD, \absolute) {
-        die "Must specify something as a path: did you mean '.' for the current directory?"
-          unless nqp::chars(path);
+        die empty-path-message unless nqp::chars(path);
 
         X::IO::Null.new.throw
           if nqp::isne_i(nqp::index(path, "\0"), -1)
@@ -56,7 +57,7 @@ my class IO::Path is Cool does IO {
           $SPEC.join($volume,$dirname,$basename), $SPEC, $CWD.Str, False)
     }
     multi method new(IO::Path:) {
-        die "Must specify something as a path: did you mean '.' for the current directory?";
+        die empty-path-message;
     }
 
     method is-absolute(--> Bool:D) {