Deprecate onerror

[Rich-Harris]

There's no value in a user-supplied onerror — it's not like onwarn where you can filter out warnings you don't want to deal with. An error is an error. The user should wrap svelte.compile in a try-catch block.

I think we should noisily deprecate it, and remove it in v3.

[Conduitry]

This should be reflected in the table of compiler options in the readme. Perhaps remove it outright?