API: #10636, changing default of to_datetime to raise, deprecating coerce in favor of errors
closes #10636 

changes `to_timedelta` API as well to similar effect.

@jorisvandenbossche updated if you want to have a look again.
 @jorisvandenbossche @shoyer @sinhrks @TomAugspurger any other comments?
 Looks good.
 Looks good!

One thing I was thinking: it would be nice if in the error message you get when there is a parsing error, that it also says something like "to coerce errors as NaN use "errors='coerce'" " if possible
 @jorisvandenbossche your last point is nice, but I think a separate PR is in order. If you would like to open an issue gr8.
 @jorisvandenbossche is there a way to limit tracebacks automatically in Sphinx (ones that are on purpose), other than using a code block

eg http://pandas-docs.github.io/pandas-docs-travis/whatsnew.html#changes-to-to-datetime-and-to-timedelta
 no, I don't think so (in any case, the `okexcept` does not do that), but that is already a long time something I wanted to fix. Will try to do that for 0.17

And for the error message suggestion, certainly, that was no blocker for merging this! 
