react: example: Check all dependents to avoid partial write

[petertseng]

Fixes #202

[petertseng]

I could write a test to deal with this, but this is quite an implementation detail. I will add the test and see what reviewers think.

[petertseng]

Test added. Example solution fails the attached test unless it has the attached code change.

failures:

---- do_not_break_cell_if_creating_compute_cell_with_valid_and_invalid_input stdout ----
    thread 'do_not_break_cell_if_creating_compute_cell_with_valid_and_invalid_input' panicked at 'Cell to update disappeared while querying', src/lib.rs:119
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    do_not_break_cell_if_creating_compute_cell_with_valid_and_invalid_input

It's not guaranteed that this will catch every single implementation that incorrectly does partial writes, but it's the best I can do, I think.

In case you wonder, this test doesn't seem necesary to add to exercism/problem-specifications#358 because most other tracks don't have to deal with cell IDs like we do; they can just have the cells reference each other (we didn't do that in Rust because mutual references are tricky).