Clarify error message

[ParadaCarleton]

julia> using Symbolics

julia> @variables n, X[1:n]
ERROR: TypeError: non-boolean (Num) used in boolean context
Stacktrace:
 [1-3] ⋮ internal
     @ Base
   [4] (::Colon)(a::Int64, b::Num)
     @ Base ./range.jl:3
Use `err` to retrieve the full stack trace.

I'm guessing the problem here is you can't have a vector of unknown size, but a newer user would probably have trouble figuring that out.