report view error and warning handlers

Before long, we'll need to have an alert system visible only in the report view for:

- errors that occur during the evaluation (resources don't load, syntax errors)
- when clicking "report view" while working we should have some sort of warning if the eval state might possibly differ from a "fresh" eval if I pass the link to someone with `viewMode=report` (this is checkable by examining the history).

There may be other errors and warnings we'll want to surface if things go wrong down the line, so having some simple component set to surface these will be generally useful.

This component set & associated state should probably handled in the editor specifically, so that we can include eval frame errors such as "the eval frame never loaded".

This all said, not pressing, just wanted to capture the thought.

The issue does not name files or tests. Start by locating the editor's report view, evaluation history, and eval-frame error handling; then clarify which errors and warnings must be surfaced and how a completed alert component should behave.

Written by the indexing model from the issue text.