Add 'keep trying' to the options for unsent messages

[Biep]

It is highly frustrating to be bound to one's mouse in order to keep clicking 'Send all' because messages keep failing - while there is a computer sitting there that would be eminently suited for such a repetitive task.

[Biep]

This is a special case (but an important one) of element-hq/element-meta#364.

[eras]

And should it eventually give up trying it, a message should be prominently shown to the user.