Pipeline Diagnostics

[dominiklohmann]

Implement a better mechanism for diagnostics from pipelines. They should supersede warnings and errors and have the following properties that our current warnings and errors do not have.

• They must be displayed in the process that started the pipeline.
• They should actively help the user see where the diagnostic came from and why it happened.

As part of working on the new diagnostics, we need to ask ourselves the following questions:

• How do we render diagnostics (warnings and errors, respectively)?
  • on the command line
  • in the frontend (e.g., should not show operators that were added by the frontend)
• How do we handle create/start time and runtime diagnostics, respectively?
• How do we report certain error classes, e.g., type clashes?
• How do we emit diagnostics in pipelines?
• How do we transport diagnostics to the frontend?
• Can we make syntax improvements to optimize for usable diagnostics?

### Definition of Done
- [x] Design new diagnostics mechanism
- [x] Design how we display diagnostics in the Terminal
- [x] Design how we display diagnostics in the Explorer
- [x] Render diagnostics in the Terminal
- [x] Render diagnostics in the Explorer
- [x] Implement the new diagnostics mechanism
- [x] Replace errors and warnings with diagnostics where appropriate

[dominiklohmann]

@annadehtiarova, @jachris and I just had a meeting on this to discuss the scope that we want to have for the beta launch, and also agreed on what we ultimately want to have.

Beta Launch

• Error diagnostics only, and only in the explorer.
• We want to add coloring initially based on the coloring used on the command-line.

Post-Beta (follow-up roadmap item to be scheduled separately)

• Display warnings: Either in the context pane, or where we currently show the data table, with a toggle to switch between warnings and errors.
• Add rich display for hints and docs.
• Visually link between warnings and the editor for annotated diagnostics.
• Render warnings and errors for the overview page.

[dominiklohmann]

We're closing this with the follow-up items moved into #71.