warn of asynchronous processes before prompting for next URL

[jidanni@xxxxxxxxxxx]

g runs the command w3m-goto-url, which is an interactive compiled Lisp
function in `w3m.el'.

The problem is, it will prompt you like everything is OK,
then only after you painfully type in the URL you want to browse, does
it finally tell you
"Cannot run two w3m processes simultaneously (Type `C-c C-k' to stop
asynchronous process)".

It should tell you that before prompting!