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This cl adds a new error message when the custom text field entry
passes the formatting check but a test DNS query fails.

Bug: 1040145
Change-Id: If25737013e942814d1f7bf0a0ac5d5c72c72e757
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2082357Reviewed-by: Eric Orth <ericorth@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Katharine Daly <dalyk@google.com>
Cr-Commit-Position: refs/heads/master@{#748985}