[JuliaLowering] Throw the same exception types as flisp
- Anything that goes wrong in macro expansion is a LoadError (previously
MacroExpansionError)
- Errors caused by bad syntax in lowering should produce ErrorException
- `Meta.lower` just returns `Expr(:error)`
It would be nice to change this later, as this limits the pretty-printing.
Assisted-by: Claude Opus 5.5
