Analyzer plugin failure at the commandline should be displayed in the terminal

[srawlins]

(I don't think they are right now)

These are all situations in which I think an error should be printed (to stdout or stderr, whatever we already do), and exit with a non-zero exit:

• an exception that occurs while setting up the shared plugin entrypoint (like a pub get failure),
• a malformed plugins section in analysis_options.yaml,
• a plugin throws an exception while computing diagnostics,

[srawlins]

CC @bwilkerson I think this should be pretty high up in the MVP priorities (sans feedback). I'm concerned that it is hard to evaluate the plugins story when the developer experience is poor. :/

[bwilkerson]

I agree. I think there are two aspects to think about: how easy it is to write plugins and how easy it is to use a plugin. This falls into the latter bucket. Users need to be able to know whether the plugins they've enabled are behaving well or whether they're causing problems, both in the IDE and from the command-line.