Better error message when DS form is invalid

[mitchell852]

For the 4 DS forms (HTTP*, DNS*, ANY_MAP*, STEERING*), there is a Show Advanced button that shows additional (mostly optional) fields for a DS. If there is an invalid entry in one of those fields, the Update button will be disabled but it is not clear why it is disabled.

Maybe some type of "This form has an error" message needs to be displayed at the top of the form when a field value is invalid.

[mitchell852]

the form has changed since this issue was created