Fix panic on Text UI without Cameras

[doonv]

Objective

Fix #11396.

Solution

Don't panic on taffy node not existing.

Plus minor warning text improvement.

[pablo-lua]

Simple changes that will probably fix the bug.

If we go about improving warnings, there are some points that I suggest a change (Maybe in a future PR):

            None => {
                if cameras.is_empty() {
                    warn!("No camera found to render UI to. To fix this, add at least one camera to the scene.");
                } else {
                    warn!(
                        "Multiple cameras found, causing UI target ambiguity. \
                        To fix this, add an explicit `TargetCamera` component to the root UI node {:?}",
                        entity
                    );
                }
                continue;
            }

In this part, we warn the user about the existence of "multiple" cameras, but this warn isn't exactly true. In fact, this function return None only if there is no cameras pointing to the PrimaryWindow. If there is no Cameras pointing to the PrimaryWindow and cameras isn't empty, the warn message isn't exactly accurate.
One possible messages is: "No camera found for PrimaryWindow, but found Cameras for other windows"

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

This should return a Result, not an Option.

[doonv]

I'll do that in a future PR.

[alice-i-cecile]

We should debounce this warning using the macro from #10808 so we don't spam the users :)

[doonv]

I did consider that but ended up not doing that because different entities could trigger that warning, but it will only trigger for the first entity.

[alice-i-cecile]

Okay, let's leave it as is for now I suppose.

[alice-i-cecile]

Feel free to clean it up further (like a result type), but I won't block on that. Fixing the crash is more important.

[doonv]

Why is this marked as a breaking change?

[matiqo15]

Why is this marked as a breaking change?

update_measure was renamed to try_update_measure

[doonv]

Why is this marked as a breaking change?

update_measure was renamed to try_update_measure

It's in a private module.

[alice-i-cecile]

Why is this marked as a breaking change?

update_measure was renamed to try_update_measure

It's in a private module.

We should remove its pub marking then to avoid further confusion.