Allow multiple errors per form element via form_set_error

It seems counterintuitive that a second call to form_set_error('foo', ...) will overwrite any existing error message set for the element foo rather than adding another error message relating to it.

I came upon this while trying to improve the form validation for path_redirect, where it's possible that a hapless user might enter a redirection "from" path that is problematic in several different ways. (Yes, the help text could be improved!)

I tried doing something like:

form_set_error('foo', form_get_error('foo') . ' Bad foo detected.');

...but this causes an error @781 in form.inc.

What I propose is that an array of errors be stored for each element, and that successive calls to form_set_error append to that array. When rendering the errors, each error is displayed in the order they were added.

Sound reasonable? Is there a common workaround for this?