Have native functions throw for invalid arguments

[mfikes]

Many of the PLANCK_*** native functions check their argument counts and types and then only process things if those checks pass. When they don't the functions typically return null.

Instead these functions could throw JavaScript Errors.

This isn't that big of a deal, since these functions are typically called via ClojureScript wrappers that pass arguments correctly, but revising the functions to throw for bad arguments seems like the right thing to do anyway.

As a concrete example, the last two calls here could throw instead of just returning nil:

cljs.user=> (js/PLANCK_IS_DIRECTORY "abc")
false
cljs.user=> (js/PLANCK_IS_DIRECTORY 123)
nil
cljs.user=> (js/PLANCK_IS_DIRECTORY)
nil