Add a hint when given --extern with an indeterminate type
Add a hint when given --extern with an indeterminate type

@ubsan brought up this relatively poor error message. This adds a
help message hinting when the problem actually is, and how to fix
it.

GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust