Explore time zone lookup errors

[pitdicker]

To have something to talk about for #1448.

The Windows code is pretty clean and has OsError and InvalidTimeZoneData as error causes.

The Unix code remains in dire need of a refactoring.
I've tried to hook it up reasonably well to the error causes InvalidTzString, TimeZoneUnknown, TimeZoneNotFound, and InvalidTimeZoneData. NoTzdb isn't hooked up yet.

[pitdicker]

The Unix code remains in dire need of a refactoring.

I did a rewrite in #1457. As it turns out the caching logic was pretty much broken beyond the most basic cases.

What I have come to see more clearly related to this PR: we have three fallbacks. Any error is just swallowed and the next fallback is tried. The last fallback is to use the UTC time zone.

That we try multiple methods to determine the time zone is only logical.
However if we determine the time zone, but its configuration turns out to be broken, I am not sure it is good to continue trying other fallbacks.
Silently falling back to UTC while the user expects a local time doesn't seem great to me. We are not limited to fitting everything in an infallible API no matter what, like libc. We already return Options or LocalResults.

I'll open an issue to discuss for 0.5.

[djc]

Silently falling back to UTC while the user expects a local time doesn't seem great to me. We are not limited to fitting everything in an infallible API no matter what, like libc. We already return Options or LocalResults.

Agreed. IIRC we tried this in the beginning but it was too big of a backwards compatibility issue. Would like to address this in 0.5.