Create a function which detects if require error is not MODULE_NOT_FOUND

[KyleAMathews]

We often in Gatsby do try/catch around requiring userland code so that if the user hasn't provided that module, we can fallback on a default version of the module.

But this causes trouble if the user has some sort of code error in that module which causes the require to fail.

We should create a function which detects if there's errors in the user code and rethrow that.