Improve build runner error collection

[daurnimator]

Currently errors from build.zig are a bit messy: to display an error message the convention is to call warn and then return a dummy value. This means that we have no ability to:

• print a header before error messages
• collect these errors into a machine printable form

Proposal:

• markInvalidUserInput should take an error message and append to an internal list of messages to display
• the special build runner would emit those messages itself

Related cleanup PR: #7640

[matu3ba]

print a header before error messages collect these errors into a machine printable form

1. Do you have any opinion how the errors should look like?
2. What use cases should be covered?
3. What is the existing tooling with tradeoffs on format usage?
4. Should this be deferred after the build runner does parallel builds or is this orthogonal?