Add warning if base type of extended derived type not found

Also don't crash when tracing procedure call chains

Fixes #577

ford/sourceform.py

@@ -1450,6 +1450,8 @@ def get_label_item(context, label):
             # extended type
             extend_type = context
             while extend_type := getattr(extend_type, "extends", None):
+                if isinstance(extend_type, str):
+                    break
                 labels[extend_type.name.lower()] = extend_type
             # vars
             labels.update(getattr(context, "all_vars", {}))
@@ -1983,6 +1985,16 @@ def correlate(self, project):
         # Match variables as needed (recurse)
         for v in self.variables:
             v.correlate(project)
+
+        if self.extends and isinstance(self.extends, str):
+            warn(
+                f"Could not find base type ('{self.extends}') of derived type '{self.name}' "
+                f"(in '{self.filename}').\n"
+                f"         If '{self.extends}' is defined in an external module, you may be "
+                "able to tell Ford about its documentation\n"
+                "         with the `extra_mods` setting"
+            )
+
         # Get inherited public components
         inherited = [
             var