System for reporting warnings (soft errors)

[DrJosh9000]

Description

• Add a new system for reporting soft errors (called warnings) through the error interface.
• Respect warnings in the various layers of the parser: rather than return immediately, accumulate warnings and return them together at the end.
• Add new warnings where some errors were previously dropped when falling back to UnknownStep, and where a pipeline is parsed with no steps.

Background

We have some cases in the parser where, when a failure occurs, parsing continues but producing a pipeline with lower semantic information:

• (Resilience) Maybe this parser has a bug, and we'd rather upload something if there's a chance the backend will accept it than block the user
• (Future compatibility) Maybe we want to introduce a new step type in the future, but allow older agents to continue to upload pipelines without knowing about them

There may be other reasons. These all boil down to "this package is not the authoritative specification for pipeline YAML".

However, silently dropping errors is not great, when they could be used to diagnose why a pipeline isn't being interpreted correctly. Without vigilance, it's easy to write YAML that looks like one thing but is parsed as another. For example:

- command: echo hello
  env:
    KEY: {"this_json":"should have been quoted"}
  branches: !main

will not parse correctly because

• the env block requires strings for values, yet the inline JSON will be parsed as a mapping (this is true here and on the backend)
• ! has special meaning in YAML when not quoted, so it will end up with branches: ''

[moskyb]

one super tiny nit, but other than that this looks awesome! great work