Evaluate making ImplementationError an actual type

[spacebear21]

@DanGould :

Making me wonder if our pub type ImplementationError = Box<dyn error::Error + Send + Sync>; type alias can be std::error::Error and 'static if we make it our own type instead, and if that would solve some problems with ffi expecting to deal with std::error::Error types

[spacebear21]

Partially addressed by #768 - I think the callsites of ImplementationError in FFI can now be updated to use a boxed error instead of downcasting to a string, then the ugly impl From<String> for ImplementationError hack may be removed.

[spacebear21]

Closed by #1029