fix: look tags up by artist name rather than by MusicBrainz id

Tag lookups asked by MusicBrainz id whenever Lidarr had one, which is always.
The reasoning was that an id cannot be ambiguous the way a name can. In practice
Last.fm's mbid index is stale and partial, and it answered "the artist you
supplied could not be found" for Devo, Escape the Fate, Blasterjaxx, Frank
Carter & the Rattlesnakes and a long tail of others -- artists whose Last.fm
pages plainly exist and carry precisely the tags the moods are built from. Devo
is tagged new wave, post-punk and 80s; Escape the Fate is tagged post-hardcore,
screamo and emocore. Both were skipped.

Ask by name first, which is the index Last.fm's own site runs on, and keep the
id only as a fallback for a name Lidarr spells differently.

Failures were also being dropped without recording the attempt, so every one of
those artists was re-queried on every subsequent pass, indefinitely. They are now
distinguished: an artist that neither key resolves is recorded as fetched with
no tags and not asked about again, while a genuine failure -- a rate limit, a
bad key -- is deliberately left unrecorded so the next pass retries it. Telling
the two apart needed the service's own error number, so LastfmError now carries
it.

tests/conftest.py

@@ -86,7 +86,13 @@ class FakeLastfm:
             return json.dumps(self._loved(query))
         if method == "artist.gettoptags":
             key = query.get("mbid") or query.get("artist", "")
-            return json.dumps({"toptags": {"tag": self.tags.get(key, [])}})
+            if key not in self.tags:
+                # What the real service says for a key it cannot resolve, which
+                # for mbids is a great many artists whose pages plainly exist.
+                return json.dumps(
+                    {"error": 6, "message": "The artist you supplied could not be found"}
+                )
+            return json.dumps({"toptags": {"tag": self.tags[key]}})
         raise AssertionError(f"unexpected method {method}")
 
     def _recent(self, query):