clearer error message for using non-booleans in boolean context

[StefanKarpinski]

julia> (Int32 :: Type{Int32}) && (Type{Int32} <: typeof(Int32))
type error: lambda: in if, expected Bool, got BitsKind

[StefanKarpinski]

Sorry, just realized this is because the left hand side returns a type, not a boolean. This works:

julia> isa(Int32,Type{Int32}) && (Type{Int32} <: typeof(Int32))
true

Changing this to be a request for better error messages.

[JeffBezanson]

Hmm... hard to do, as && is converted to if very early on.