[Breaking] Change error from directive to keyword.

[blishko]

This is an updated version of the original PR #11218.

More context:
Plans for keywords in 0.9: #14770
Discussions specifically about error as keyword: #11743, #11859, #14770 (comment).

[blishko]

What's the point of this test?
I believe error E; as a statement in a function should now cause a parser error.
If this test is supposed to get past the parsing stage then we need to remove this statement.

[nikola-matic]

What's the point of this test?

I think we discussed it in chat, but yes, the point is to test the behaviour of the parser without the subsequent analysis step (that would absolutely fail all of these).

If this test is supposed to get past the parsing stage then we need to remove this statement.

Yup, that's exactly right.

[nikola-matic]

I think it would be nice for us to have a protocol in place when opening PRs the target the breaking branch; e.g. prefix PRs with [breaking] or something like that, so that it's easier to scan for these when looking at the PR list.

[blishko]

I think it would be nice for us to have a protocol in place when opening PRs the target the breaking branch; e.g. prefix PRs with [breaking] or something like that, so that it's easier to scan for these when looking at the PR list.

That's a good idea. Sorry, I got distracted and did not label this PR as breaking.
We can use both label and title prefix.