Replace DiagnosticsProviding protocol with concrete type

[elizablock]

As discussed in #1318, a nicer design for providing diagnostics via thrown error is to expose a concrete error type:

public struct DiagnosticsError: Error {
  public var diagnostics: [Diagnostic]
}

This soft-asserts in init if none of the diagnostics has severity == .error, and removes the burden from users of creating their own conforming error type just to throw diagnostics.

[elizablock]

@swift-ci please test

[ahoppen]

Thank you. Looks good to me.