Make it possible for a user to understand why their blueprint doesn't work

[jleibs]

At the moment our heuristics generally create space-views that are designed to be functional.

However, Blueprint is likely to introduce many new means of users creating blueprints that don't actually do what the user intended.

For example:

• Creating query expressions that don't match the entity tree
• Using an origin that precludes visualizing the added content
• Overriding components that don't match the visualizer

We need to make it easy and obvious to figure out what's going wrong when things have been explicitly added to a blueprint but don't have any impact.

[emilk]

Related to:

• https://github.com/rerun-io/rerun/issues/4304

[jleibs]

Possible small improvements:

• Better text when nothing is found?
• When there are multiple entity expressions, warn if any single entity expression doesn't contribute content.