Improve error message when unpacking in loop misses parentheses

[Tortar]

Now:

julia> [x for x, y in [[1,2], [2,3]]]
ERROR: ParseError:
# Error @ REPL[10]:1:9
[x for x, y in [[1,2], [2,3]]]
#       └ ── invalid iteration spec: expected one of `=` `in` or `∈`
Stacktrace:
 [1] top-level scope
   @ none:1

the suggestion seems off because the in is already there, it should somehow hint you to the right direction e.g. putting (x, y).

Actually, but maybe this is controversial, a syntax like this seems good to me, and maybe it could be allowed, in Python it works both with and without parenthesis and also in Julia x, y = [2, 3] already works so it seems good for consistency.