time: return informative errors when failing to load timezone data.

If we cannot load timezone information for a reason other than the
zoneinfo file not existing, return it since that will be much more
useful in debugging failures than "unknown time zone XYZ".

Fixes #9723.

Change-Id: I3aa5774859cec28e584d16bcc1fef0705d95288c
Reviewed-on: https://go-review.googlesource.com/3984


Reviewed-by: Ian Lance Taylor <iant@golang.org>