Provide a clearer error for at-threads for a, b (#34099)

Currently, nested loops specified as `for i = 1:2, j = 1:2` fail when
passed to `at-threads`, since the macro assumes the expression is a
single `for` loop. This change just provides a more explicit error
message in this case.

Author: ararslan

base/threadingconstructs.jl

@@ -90,7 +90,11 @@ macro threads(args...)
         throw(ArgumentError("need an expression argument to @threads"))
     end
     if ex.head === :for
-        return _threadsfor(ex.args[1], ex.args[2])
+        if ex.args[1] isa Expr && ex.args[1].head === :(=)
+            return _threadsfor(ex.args[1], ex.args[2])
+        else
+            throw(ArgumentError("nested outer loops are not currently supported by @threads"))
+        end
     else
         throw(ArgumentError("unrecognized argument to @threads"))
     end

test/threads_exec.jl

@@ -705,3 +705,10 @@ let a = zeros(nthreads())
     _atthreads_with_error(a, false)
     @test a == [1:nthreads();]
 end
+
+try
+    @macroexpand @threads(for i = 1:10, j = 1:10; end)
+catch ex
+    @test ex isa LoadError
+    @test ex.error isa ArgumentError
+end