report view error and warning handlers

[hamilton]

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.