Move SysPropError to rustutils library.

This was being duplicated in every generated Rust system property
library, which meant that they were different types.

Also added `Error` implementation, and moved PropertyWatcherError to
submodule.

Bug: 270547306
Test: Built libplatformproperties_rust
Change-Id: Ib3698b531cdad49e94fa69a75f3677ad9c221c00
2 files changed