Implement std::error::Error for TemporalError

[LeandroVandari]

Currently, TemporalError does not implement std::error::Error, which makes TemporalResult incompatible with ecosystem crates for error conversions, such as anyhow.

The fix is a simple one-line addition to implement the error trait, since TemporalError already implements Display and Debug.

[nekevss]

Thanks for the contribution!