Show errors when submitting too many answers

This could be the case when JavaScript is disabled.

Note that, in `Poll/WebVote` we're calling `given_answers` inside a
transaction. Putting this code before the transaction resulted in a test
failing sometimes, probably because of a bug that might be possible to
reproduce by doing simultaneous requests.

config/locales/en/general.yml

@@ -579,6 +579,7 @@ en:
     final_date: "Final recounts/Results"
     form:
       vote: "Vote"
+      maximum_exceeded: "you've selected %{given} answers, but the maximum you can select is %{maximum}"
     index:
       filters:
         current: "Open"

config/locales/es/general.yml

@@ -579,6 +579,7 @@ es:
     final_date: "Recuento final/Resultados"
     form:
       vote: "Votar"
+      maximum_exceeded: "has seleccionado %{given} respuestas, pero el máximo que puedes seleccionar es %{maximum}"
     index:
       filters:
         current: "Abiertas"