Improve error message for extends

[agoose77]

Addresses #2405

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[agoose77]

Working on this PR surfaced something I've noticed implicitly — our story for error handling (and probably the TUI more generally) could benefit with being clearer.

e.g. we have

• fileWarn for pure transforms
• session.log and addWarningForFile for session transforms
• thrown exceptions

In general, the core of MyST should be very tolerant to errors and warn where possible. But there are things like "broken config" that aren't really ignorable. We need a nicer way to handle such situations and early exit