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).
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