Return Error If Overflow

[YoshikiTakashima]

Fixes #682 by allowing i64 wrapping during negation.

This allows seconds = i64::MIN to pass with debug checks enabled.

[LucioFranco]

I wonder if we should actually return an error rather than wrapping. What do you think?

[YoshikiTakashima]

So with seconds = i64::min, the function already returns TimestampError::OutOfSystemRange(orig_timestamp).

I also made one that makes this error return more explicit, although it's much more verbose.

[LucioFranco]

LGTM! Thanks!