Space out errors and warnings for readability

[sezna]

I am working on #308, and I'm noticing that when a lot of errors are generated, they can be very difficult to visually differentiate.

This PR adds (somewhat primitive) visual boundaries between the errors/warnings:

I don't know if this is the best way of doing this, and I'm open to feedback and/or using an alternate separator. But I do think this improves the UX a bit.

[sezna]

I guess --> error is the separator already, but visually, it all blends together to me. I'm open to being overridden here, I suppose.

[adlerjohn]

Why was the lockfile updated?

[sezna]

I'm not sure what exactly changed, but I just reset the lockfile and rebuilt, and it got updated again. Maybe the toml changed on master but the lockfile wasn't committed?

[adlerjohn]

Ah makes sense. Can you make a separate PR to update the lockfile in isolation?

[eureka-cpu]

lgtm, can't think of a better way off the top of my head.

[nfurfaro]

I think this change would be helpful. A possible improvement might be to either:

1. change the color of the ----
2. use more ----'s ! ie: Each error in its own box. Not sure if that would look crowded though.

[mitchmindtree]

Looks fine to me, I have no major preference either way :)