Improve warning for missing IOable attributes

desc/io/hdf5_io.py

@@ -124,7 +124,24 @@ def read_obj(self, obj, where=None):
         for attr in obj._io_attrs_:
             if attr not in loc.keys():
                 warnings.warn(
-                    "Save attribute '{}' was not loaded.".format(attr), RuntimeWarning
+                    f"Save attribute '{attr}' was not loaded from the input file.\n"
+                    "This is likely because the file from which you are loading the "
+                    f"Python object '{obj.__class__.__name__}' was created "
+                    f"prior to the time '{attr}' "
+                    f"became an attribute of objects of the class '{obj.__class__}'.\n"
+                    "\n"
+                    "Note to developers: Add 'def _set_up(self)' as a method to class "
+                    f"{obj.__class__}\n"
+                    "(or the superclass where this new attribute is assigned) that "
+                    f"assigns some default value to '{attr}' for old objects.\n"
+                    "This method will be called automatically when a file is loaded.\n"
+                    "\n"
+                    "This warning will continue to be raised until the file is saved "
+                    "with an updated object, even if the _set_up method assigns "
+                    "the missing attribute correctly.\n"
+                    "Our testing suite will fail on warnings, so developers may want "
+                    "to comment out this warning until the input files are updated.\n",
+                    RuntimeWarning,
                 )
                 continue
             if isinstance(loc[attr], h5py.Dataset):