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).

spec/system/admin/dashboard/actions_spec.rb

@@ -63,7 +63,8 @@ describe "Admin dashboard actions" do
 
     scenario "Renders create form in case data is invalid" do
       click_button "Save"
-      expect(page).to have_content("errors prevented this Dashboard/Action from being saved.")
+
+      expect(page).to have_content("error prevented this Dashboard/Action from being saved.")
     end
   end