Using form_set_error() to display error messages, the first argument is the name of a field.
$form['height'] = array(
'#type' => 'textfield',
'#title' => 'Height',
); Along with:
form_set_error('height', 'Error with height.'); 
Normally this is simple to define, but when using nested form items within fieldsets, this doesn't highlight the right item.
If the form was changed to nest the items:
$form['new'] = array(
'#type' => 'fieldset',
'#title' => 'New Image Size',
'#tree' => true,
);
$form['new']['height'] = array(
'#type' => 'textfield',
'#title' => 'Height',
); Using:
form_set_error('height', 'Error with height.'); 
Notice the height is no longer highlighted?
The reason is that the field argument requires some formatting to make it work with nested forms.
Using a stupid '][' to tokenise the fieldname, we can specify which item to highlight.
form_set_error('new][height', 'Error with nested height.'); 
This gives us the proper feedback we wanted.
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