Better errors

[bakkot]

For example, having multiple linebreaks in a <p> should not abort the build with "Error: expecting EOF, got parabreak".

At the very least we need to start wrapping those and providing location information, because the errors at the moment are totally useless. But also we should clean up the output to suggest what's actually wrong. And in the above case it shouldn't be a hard error at all, just a lint warning.

[bakkot]

Thinking about this some: I'd like to use the same format for other errors as we do for linting errors. Those errors are not identified until later in the pipeline. But the eslint error formatters don't support streaming: specifically, they always add a "1 error found." summary at the end, which would be weird to see multiple times. I'm not entirely sure how to handle it. Here's some possibilities, in roughly descending order of how much I like them

• drop support for arbitrary formatters and reimplement the codeframe formatter
• drop support for arbitrary formatters and manually modify the output from codeframe to not have the summary
• support arbitrary formatters but try to tweak their output anyway
• support arbitrary formatters and just not worry about the confusing output
• not report any errors until the end of the build, so we can report them all at once

Maybe there's some other alternative?

Edit: went with another alternative, which is to just print errors twice: a quick summary when encountered and, if using --verbose, all the errors together and formatted at the end of the bulid.