progress: would like to indicate error before completion

[virtuald]

Is your feature request related to a problem? Please describe.

Let's say I have 100 pieces of a task, each piece takes 10 seconds to accomplish, and I can do them concurrently. If one of them fails, I'd like to tell the user that one of them has failed, but the others are still making progress.

Describe the solution you'd like

It would probably be best to let the tracker continue, and update the tracker message to indicate that an error occurred. Currently Tracker.Message is public, but (even though not documented as such) probably shouldn't be written after a tracker is started because string assignment isn't guaranteed to be atomic (https://groups.google.com/g/golang-nuts/c/UWJlS8mPTlM).

Adding a mechanism to update the tracker message (and/or color) would work nicely. The user would still need to finalize the tracker via MarkAsErrored. What do you think?

... yes, I know I added MarkAsErrored, but clearly I didn't actually think through what I really wanted before doing that. :)