Assert that storage key is defined.

[shaper]

This came up as an attempt to split on an undefined string while initially exploring Noe's prototype Social recipes. The recipe file has:

map 'FRIENDS_PROFILE_posts' #friends_posts as posts

and this ends up finding no storage key in Arc.instantiate. The recipe is rather kooky but at minimum having a more informative message seemed worth considering. Ref:

https://github.com/shaper/arcs-stories/blob/master/social/Social/Social.recipes#L25

[shaper]

@piotrswigon this is an example where a recipe (written by someone else) that I started looking at was failing with no useful developer facing info. It was just throwing an exception attempting to parse the storage key when the string was undefined.

[piotrswigon]

Thanks for doing this, though I'm not sure I'm an approved reviewer for arcs/ yet, let's see what happens :)