Improve how backend errors are shown

[rikimaru0345]

This isn't nice:

Instead, we should

1. not emit the same error multiple times (maybe just parse/compile once first, and if that fails return, instead of letting each consumer compile the code itself and then have them report the error separately)
2. collect errors in the frontend and have a proper "collection"/"log" where the user can view the errors and clear them (because there are some legitimate scenarios where multiple errors might be returned).

[twmb]

cc @ivpanda for some ideas here

[ivpanda]

I thought this would be a simple one, but (unsurprisingly) it isn't — @twmb (or @rikimaru0345 ), can you give me an example of an instance of that legitimate scenario where multiple errors are returned (and are those errors all the same)? I also have a couple of follow-ups:

1. Do we have any control over these errors and their wording?
2. Do we know what users need to do in each case, or is it best to just present the error and trust that they'll know what to do?
3. Are there any other actions a user may want to take on this other than clearing it and/or fixing the error?
4. Are there any errors that are irrelevant?