Add a special-cased error message for returning a Result from main, to indicate improper try! usage

[bstrie]

To beginners it's surprising that try! can only be used within a function with a Result return type, which in particular means it cannot be used from main. In the past we've mulled over language-level support for solving this problem but ultimately maybe we just need a better error message. How hard would it be to just detect try! being called from within main and give a nice and specific error message?

[steveklabnik]

This is a duplicate of #27112