Error message for `pointerof` should be more helpful

[straight-shoota]

When passing pointerof a value that doesn't have a pointer location, it errors with the message can't take address of #{expression}.

This is not very helpful because it requires an understanding of how pointerof works and what kind of expression it is supposed to receive. This can be challenging for newcomers (cf. https://forum.crystal-lang.org/t/sdl-event-and-userevent-questions/8114/6?u=straight-shoota).

Instead of only stating that taking an address of the given expression doesn't work, it could be useful to state what would work: The expression must be a variable (local, instance, class) or a constant.
Perhaps it would also be good to mention the type of the expression.

Here's a suggestion for rewording the message:

# old
pointerof(1) # Error: can't take address of 1

# new
pointerof(1) # Error: can't take address of `1` because it's a NumberLiteral. `pointerof` expects a variable or constant.

[crysbot]

This issue has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/sdl-event-and-userevent-questions/8114/9