https://wicg.github.io/import-maps/#register-an-import-map

Currently preparation-time document check is done at Step 6, but to align with https://html.spec.whatwg.org/multipage/scripting.html#execute-the-script-block, the check should be before firing error event at Step 1.

(This is a quite low-priority issue found during Chromium implementation investigation for an unrelated non-import-map issue. I'll create a PR/impl fix when I have a spare bandwidth)