Should vuex mapState print error message

[balazser]

What problem does this feature solve?

I got the following error: Uncaught TypeError: Cannot convert undefined or null to object
then realized I invoked the ...mapState(NSP_CONCEPT) without a second parameter.
It took some time while I figured out, would have been great, if there is a condition to don't fail or print out a message.

What does the proposed API look like?

print a message, or put a condition into the function

[ktsn]

fixed via #1297