# Objective
Fixes#15394
## Solution
Observers now validate params.
System registry has a new error variant for when system running fails
due to invalid parameters.
Run once now returns a `Result<Out, RunOnceError>` instead of `Out`.
This is more inline with system registry, which also returns a result.
I'll address warning messages in #15500.
## Testing
Added one test for each case.
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## Migration Guide
- `RunSystemOnce::run_system_once` and
`RunSystemOnce::run_system_once_with` now return a `Result<Out>` instead
of just `Out`
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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Zachary Harrold <zac@harrold.com.au>