Per-category field validation:

• Can specify a set of additional metadata fields that are required for an example category. This is useful for tributaries that specify additional fields for specific example types.
• It's not clear how much this will be used beyond IAMPolicy, so I used a simple hardcoded dict, but this lays some groundwork for expanding this to a configurable file specified in the tributary.

Improved some error messages to help metadata authors:

• Caught YAML parse error so we write a helpful message instead of the stack trace.
• Added tips for fixing some other common errors where the fix might not be obvious.

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