Define and use an abstract ErasedReportedError type instead of ()

[fmease]

Heavily inspired by rustc.

• 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.
• Defined in crate::diagnostic for now as ErrorReported
  • 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
• Change crate::error's Result definition to default E to ErrorReported instead of ()
• Remove the #![allow(clippy::result_unit_err)] in the crate roots as it will no longer be necessary
• 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))
• 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)
• Add documentation to ErrorReported (in [WIP] Fix exposure related bugs #137)
• Bugs in the current implementation
  • 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 6cb2bfe

[fmease]

Renamed from ErrorReported to ErasedReportedError.