Cryptic *missing Update implementation* compiler errors

[MichaReiser]

A missing Update implementation for a salsa struct field or a query return type can read rather cryptic and it's not clear what a user should do (e.g. implement Update or remove the &'db reference.

I wonder if doc.rust-lang.org/reference/attributes/diagnostics.html#the-diagnosticon_unimplemented-attribute is useful here? (unsure given we go through the fallback trick)
@Veykril