Fix bulk fill for usage

Previously, if there existed any notes in the target deck that didn't
have one of the hanzi fields, the `get_hanzi` step would fail as a
result of `cleanup` raising a `ValueError` if it received `text=None`.

This PR modifies the logic so that presence of a `hanzi` field is
verified before getting the `hanzi` itself.

It also modifies the behavior such that cards that do not have usages
filled are marked as failed. Otherwise, the user doesn't know why many
cards did not have usages filled.

Relevant to jdlorimer#177.

chinese/fill.py

@@ -465,7 +465,8 @@ def bulk_fill_usage():
             <b>Cards with no Sentences added:</b> %(not_filled)d<br>
             <b>Failed:</b> %(failed)d'''
 
-    fields = config.get_fields(['usage'])
+    target_fields = config.get_fields(["usage"])
+    hanzi_fields = config.get_fields(["hanzi"])
 
     if not askUser(prompt):
         return
@@ -476,24 +477,30 @@ def bulk_fill_usage():
     n_notfilled = 0
     failed_hanzi = []
 
-    note_ids = Finder(mw.col).findNotes('deck:current')
+    note_ids = mw.col.findNotes("deck:current")
     mw.progress.start(immediate=True, min=0, max=len(note_ids))
 
     for i, note_id in enumerate(note_ids):
         note = mw.col.getNote(note_id)
         copy = dict(note)
-        hanzi = get_hanzi(copy)
 
-        if has_any_field(copy, fields) and hanzi:
+        # Ensure note type has hanzi present before trying to get
+        hanzi = None
+        if has_any_field(copy, hanzi_fields):
+            hanzi = get_hanzi(copy)
+
+        if hanzi and has_any_field(copy, target_fields):
             n_processed += 1
 
             try:
-                if all_fields_empty(copy, fields):
+                if all_fields_empty(copy, target_fields):
                     result = fill_usage(hanzi, copy)
                     if result:
                         n_updated += 1
                     else:
                         n_notfilled += 1
+                        n_failed += 1
+                        failed_hanzi.append(hanzi)
             except:
                 n_failed += 1
                 failed_hanzi.append(hanzi)