Running workflow when worker is down - error displayed

[tsurdilo]

To reproduce, start a workflow exec and shut down your workers.
I am not sure why this error is displayed, seems no info is really missing.

[Alex-Tideman]

@tsurdilo We show this error because it's a running workflow but it has no running workers. Would you not want to see this in that situation or would like to see different copy? @canvanooo

[lorensr]

Here's a suggestion: #604

[stevekinney]

Closing in favor of #604.