docs: expand and improve error handling documentation (#96)

[pylrn]

Description

Fixes #96

This PR expands and clarifies error handling documentation across the SDK:

• Exception Hierarchy: Documents the full custom exception hierarchy (AssemblyAIError, TranscriptError, SyncTranscriptError, RedactedAudioIncompleteError, RedactedAudioExpiredError, RedactedAudioUnavailableError, and RealTimeError/StreamingError) along with their respective attributes.
• Handling Patterns: Provides concrete examples for differentiating error cases using try...except, validating transcript statuses (aai.TranscriptStatus.error), inspecting HTTP status codes, and accessing raw response headers/body via Client.get_default().last_response.
• Sync STT Errors: Documents SyncTranscriptError.error_code strings and retry_after timing for 429/503 responses.
• Streaming Errors: Documents how WebSocket streaming errors are dispatched to RealTimeEvents.Error handlers and mapped via RealTimeErrorCodes.
• Table of Contents Sync: Updated the Table of Contents anchor to match the expanded section.

[ccampbell-aai]

Thank you for taking the time to give detail on these exceptions! I've left a few minor suggestions and improvements but overall this is very well done.

[ccampbell-aai]

Thanks again for this! I pushed a commit to your branch addressing the three review comments (RealTimeError inheritance note, TranscriptError wording, and assemblyai.types. paths for the RedactedAudio*Error classes) so we can get this merged.