When JSON parsing of inbox output fails, the code falls back to plain
text mode. However, the error from the fallback `gt mail inbox` command
was being silently ignored with `_`, masking failures and making
debugging difficult.
This change properly captures and returns the error if the fallback
command fails.
Co-authored-by: Gastown Bot <bot@gastown.ai>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
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