timers: fail early when callback is not a function

`setTimeout()`, `setInterval()` and `setIntermediate` currently
throw errors when receiving non-function objects as their first
argument, but only do so when trying to execute the callback,
i.e. after the waited time has passed. This may complicate
debugging when a lot of calls to `setTimeout()`/etc. are involved,
so failing as early as possible seems like a good idea.

`setTimeout()` historically ignored an falsy first
argument, while the other functions do not and throw instead.
This patch changes this behaviour to make all three match and
adds remarks in the corresponding documentation.

PR-URL: https://github.com/nodejs/node/pull/4362


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>