Return custom result/error types

[thewh1teagle]

Currently we use eyre::{Result,bail} but but Rust standard is to return custom types from crates.
I'm not sure we gonna use it only because it's standard because it requires more work. so need to see if it's worth it other than the fact.
eg. How really useful to external library that uses sherpa-rs is to know that the error is InvalidPointer returned from sherpa... seems like not much...