w3m-goto-url on dodgy url file:/foo.html

[Kevin Ryde <user42@xxxxxxxxxx>]

With the current cvs, evaluating

    (w3m-goto-url "file:/foo.html")

gets an error

    Debugger entered--Lisp error: (wrong-type-argument stringp nil)
      file-directory-p(nil)
      w3m-goto-url("file:/foo.html")
      eval((w3m-goto-url "file:/foo.html"))

The url of course should be "file:///foo.html", but it might be good if
w3m-goto-url either allowed a mistake like that, or gave a more
informative error message than "stringp".

I had this in my WWW_HOME environment variable, I think for lynx, maybe
for old mosaic even.  Lynx allows it, but I think an error might be
best, so as not to encourage anyone to use an incorrect syntax.