Add more context to missing key error

https://github.com/EspressoSystems/espresso-sequencer/blob/c78752de617f58e9d86f4347013714992f018f81/sequencer/src/options.rs#L249C1-L250C1

We want errors that take the key path to be more descriptive. Right now they are just

```
Error: No such file or directory (os error 2)

Caused by:
    No such file or directory (os error 2)
```