Don't show a tuple for PointerToNowhere's repr.

Possibly we should later also show which parts of the pointer *do*
exist, but for now this is a start.

Author: Julian

referencing/exceptions.py

@@ -42,3 +42,6 @@ class PointerToNowhere(Unresolvable):
     """
 
     resource: Any
+
+    def __str__(self):
+        return f"{self.ref!r} does not exist within {self.resource.contents!r}"

referencing/tests/test_core.py

@@ -353,6 +353,7 @@ def test_lookup_non_existent_pointer(self):
             ref="/foo/bar",
             resource=resource,
         )
+        assert str(e.value) == "'/foo/bar' does not exist within {'foo': {}}"
 
     def test_lookup_non_existent_pointer_to_array_index(self):
         resource = Resource.opaque([1, 2, 4, 8])