better error message for e.g. `convert(Tuple{NTuple{2, Int}, Int}, (3,3,3))`

[goretkin]

The convert methods are defined using the structural recursion on tuples, so the error message comes from a nested call:

julia> convert(Tuple{NTuple{2, Int}, Int}, (3,3,3))
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type Tuple{Int64,Int64}

It might be better to produce the following error message, if possible.
ERROR: MethodError: Cannot `convert` an object of type Tuple{Int64,Int64,Int64} to an object of type Tuple{Tuple{Int64,Int64},Int64}

[vtjnash]

I'd attempted this in #32427, but had gotten hung up on a type-system bug that made it segfault when I tried to detect this case. I think I've found a workaround.

[ViralBShah]

With julia master:

julia> convert(Tuple{NTuple{2, Int}, Int}, (3,3,3))
ERROR: MethodError: Cannot `convert` an object of type 
  Tuple{Int64,Int64{},Int64} to an object of type 
  Tuple{Tuple{Int64, Int64},Int64{}}

[vtjnash]

Fixed by #32427