missing error handling in descend entry for non-types

[vtjnash]

julia> descend(show, (1,))
ERROR: MethodError: no method matching widenconst(::Int64)

Inference assumes you have handled this case already before entry, usually by sanitizing with one of the entry functions (signature_type or to_tuple_type)

julia> code_lowered(show, (1,))
ERROR: argument tuple type must contain only types
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] to_tuple_type(t::Any)
   @ Base ./reflection.jl:990
 [3] signature_type(f::Any, argtypes::Any)
   @ Base ./reflection.jl:1000