Fix a bug in stack's DimensionMismatch error message (#54033)

`stack` does not require that the inner iterator defines `axes`, but the
code to assemble an error message assumed this.

Found here:
https://discourse.julialang.org/t/reduce-hcat-is-type-unstable/112800/3

Author: mcabbott

base/abstractarray.jl

@@ -3007,7 +3007,7 @@ end
 @inline function _stack_size_check(x, ax1::Tuple)
     if _iterator_axes(x) != ax1
         uax1 = map(UnitRange, ax1)
-        uaxN = map(UnitRange, axes(x))
+        uaxN = map(UnitRange, _iterator_axes(x))
         throw(DimensionMismatch(
             LazyString("stack expects uniform slices, got axes(x) == ", uaxN, " while first had ", uax1)))
     end

test/abstractarray.jl

@@ -1784,6 +1784,9 @@ end
     @test_throws ArgumentError stack([1:3, 4:6]; dims=3)
     @test_throws ArgumentError stack(abs2, 1:3; dims=2)
 
+    @test stack(["hello", "world"]) isa Matrix{Char}
+    @test_throws DimensionMismatch stack(["hello", "world!"])  # had a bug in error printing
+
     # Empty
     @test_throws ArgumentError stack(())
     @test_throws ArgumentError stack([])