Unskip and update tests for checkbox with default.
Update FormHelper::error() to reflect current implementation.

Use templates for HTML generation instead of attribute/array smashing.
This will break existing use cases, but I honestly doubt people use many
different kinds of error formatting and probably ended up accepting
defaults, or copy and pasting formatting options everywhere.
Remove tagIsInvalid()

With the addition of error templates, I think this method is mostly
useless, and can be replaced with `$this->Form->context()->hasError()`
where it is really required.
Update isFieldError() to use the context object.
Update doc blocks.
Merge pull request #2875 from markstory/3.0-form-error

3.0 FormHelper error()