proposal: Use object not found response http code 404 when no object is found

It's currently a bad request (400) which confuses the readability in the metrics dashboard [1]

It's probably making some clients (as in real people) of the objstorage go bonkers too...

[1] https://grafana.softwareheritage.org/goto/h7SosBnHz?orgId=1

swh/objstorage/api/server.py

@@ -55,7+55,7 @@
 
 
 class ObjStorageServerApp(RPCServerApp):
     client_exception_classes = (
         Error,
         NoBackendsLeftError,
         ObjCorruptedError,
@@ -86,7+86,7 @@
 objstorage = None
 
 
-@app.errorhandler(Error)
+@app.errorhandler(ObjNotFoundError)
 def argument_error_handler(exception):
+    return error_handler(exception, encode_data, status_code=404)
+
+
+@app.errorhandler(Error)
+def objnotfound_error_handler(exception):
     return error_handler(exception, encode_data, status_code=400)