Add "Failed to send" error indicator to failed replies #638
Description
Follow-up from #365. We are currently color-coding the line at the bottom of failed replies, but giving no other indicator to the user that the reply failed.
As a first step, we should implement the "Failed to send" indicator as shown in this design:
https://scene.zeplin.io/project/5c807ea562f734bd2756b243/screen/5d5b1878d376a09c09d57c5a
NOTE: The "Retry" and "Delete" actions, the message bubble, and the error indicators in the source list, are out of scope to resolve this issue.
In any situation in which the color-coding of the line at the bottom is restored to its original color or to an active state (e.g., if a reply is being retried), the "Failed to send" indicator should be removed.