Improve brimcap error handling

[nwt]

Zui displays a Node exception if brimcap exits without reading its standard input to EOF. Display an error toast with details instead.

Update, here's how the branch looks now.

brimcap_error.mp4

Closes #2926

[philrz]

I tested out the branch with the pcap from #2926 (comment). The error presentation is improved enough that I'd consider this a fix for #2926.

[philrz]

@jameskerr: The e2e test looks good. FYI, I made a temporary branch based on this branch to which I merged main so I could have the knobs available in my improved e2e.yml. With that I did this Actions run that shows your e2e test running reliably on all OSes in CI.

At this point I think I'm good with this one merging, but since @nwt was already involved in this code and I know the Zui/Brimcap error channel has some complexities, I'd feel better if he had a chance to give it a 👍 first. I guess since he opened the PR he technically can't be the one to click "Approve" but I'll wait for him to give the word somewhere and then I'll do the clicking.

[philrz]

@nwt and I talked offline and he didn't feel he had more to add to the review, so I'm hereby giving it the 👍.