Define and use an abstract `ErasedReportedError` type instead of `()`

[fmease]

Heavily inspired by rustc.

• [x] The goal is to prevent the case where a pass fails (currently with Err(())) but no error is reported (via Diagnostic::error().report) which leads to a failed assertion and a reported internal compiler error in main.
• [x] Defined in crate::diagnostic for now as ErrorReported
  • [x] Ideally, no “plain” constructor but returned by Reporter::report/Diagnostic::report. However, this won't work with crate::resolver::scope::RegistrationError::DuplicateDefinition which doesn't immediately result in a reported diagnostic by design. Which means we probably need to expose some constructor named sth. like ErrorReported::promise_error_will_be_reported() ErrorReported::error_will_be_reported_unchecked unless we find a way too make this type-safe
• [x] Change crate::error's Result definition to default E to ErrorReported instead of ()
• [x] Remove the #![allow(clippy::result_unit_err)] in the crate roots as it will no longer be necessary
• [x] Require Error variants of possibly erroneous (→ PossiblyErroneous) data to hold/be contstructed with an ErrorReported changing PossiblyErroneous::error to take an _: ErrorReported as an argument. ErrorReported has to impl Clone because AST nodes are Clone but that is not an issue. Discarding is also not a problem right now (since a positive error count leads to a failing exit status even without an ErrorReported (maybe we want to change it making it more strict))
• [x] Store ErrorReported in Health::Tainted to be able to make Health and Outcome combinators correct (i.e. remove their calls to ErrorReported::error_will_be_reported_unchecked)
• [x] Add documentation to ErrorReported (in #137)
• [ ] Bugs in the current implementation
  • [x] Diagnostic::{warning,debug}().report(reporter) also yields an ErrorReported even though only a warning or debug message, respectively, was issued. ErrorReported should only be obtainable by ::{error, bug}()
  • [ ] SilentReporter yields a ErrorReported. Instead, it should return () or similar. We can achieve this (I think) by making the Reporter enum a trait and adding an assoc type Output

[fmease]

Halfway implemented via 6cb2bfe43db9604727f5ed6a1ae9828ee34ddc86

[fmease]

Renamed from ErrorReported to ErasedReportedError.