Throw on non-promotion in fallback UnitRange constructor

[jishnub]

This avoids a StackOverflowError in the fallback UnitRange constructor, and throws an error if the promotion does not change the types of the arguments.

On master

julia> UnitRange('a', 'b')
ERROR: StackOverflowError:
Stacktrace:
 [1] UnitRange(start::Char, stop::Char) (repeats 79984 times)
   @ Base ./range.jl:401

After this

julia> UnitRange('a', 'b')
ERROR: promotion of types Char and Char failed to change any arguments
Stacktrace:
 [1] error(::String, ::String, ::String)
   @ Base ./error.jl:44
 [2] sametype_error(input::Tuple{Char, Char})
   @ Base ./promotion.jl:417
 [3] not_sametype(x::Tuple{Char, Char}, y::Tuple{Char, Char})
   @ Base ./promotion.jl:411
 [4] UnitRange(start::Char, stop::Char)
   @ Base ~/julia/base/range.jl:409
 [5] top-level scope
   @ REPL[9]:1