Update docs for Twirp Errors

[marioizquierdo]

Update docs for Twirp Errors:

• Organize for better readability: Overview; Error codes; Server side; Client side; Metadata.
• Update examples using new error constructors and idioms.
• Show common error codes first, and include a link to see all other codes in the spec.
• Improve links to source code on relevant examples.
• Explain how vanilla (non-twirp) errors are wrapped as internal errors with more detail.

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