Don't count errors for the same field twice

The number of errors in a form includes several errors for the same
field. For example, if a title is mandatory and has to have at least 5
characters, leaving the title blank will result in two errors. So users
will be invited to look for two errors, but they'll only find one field
with errors.

So it's a bit more intuitive to show as many errors as fields having
errors.

Note we're excluding errors on `:base`, which is a bit of a hack for
errors in association fields. For example, if the title of one
translation is not present, `resource.errors.messages` will contain two
elements: one for the translation's title, and one for the `base` field.
This resulted in the count of errors being 2 when there was only one.

Also note I haven't found a way to count errors on all `has_many`
relations. That is, if two translations have a missing title field, only
one error will be mentioned in the message (as it did before this
commit).

app/views/shared/_errors.html.erb

@@ -5,7 +5,9 @@
     </button>
 
     <strong>
-      <%= pluralize resource.errors.count, t("form.error"), t("form.errors") %>
+      <% errors_count = resource.errors.messages.reject { |attribute| attribute == :base }.count %>
+
+      <%= pluralize errors_count, t("form.error"), t("form.errors") %>
 
       <% if local_assigns[:message].present? %>
         <%= message %>