Add errors_on helper for inline errors on single attributes

For your consideration, this adds an `errors_on` helper. Suppose you have a big form that creates a project and at the bottom of the form you can add any number of tasks within that same form. You are also inlining all errors on the form fields (the default). Upon submission, you want to display an "inline" error right where tasks are entered unless at least one task has been added. The helper would allow you to add  `<%= f.errors_on :tasks %>`. If errors are present on `:tasks`, then those errors would be automatically displayed wrapped in a div with the `alert` and `alert-danger` classes.