recognise all possible $foo tags, error on unsupported ones

[gregwebs]

as suggested in the google group: http://groups.google.com/group/yesodweb/browse_thread/thread/abfe006a1573be9d/3ed101295c7fca16?lnk=gst&q=hamlet+foo#3ed101295c7fca16

<ul>
   $for al <- allAuthLevels
       <li>${show al}

However, that fails because it should be $forall, not $for, which causes
the later use of `al' to be undefined.

If this were to fail due to $for being unrecognised, that would be much
better, because it means that version n+m of hamlet files would fail to
compile on version n of hamlet when a new directive has been added. (ie.
better forward compatibility).

[snoyberg]

Let's implement this in the next major release. Doing so could in theory break existing code (e.g., any line starting with a dollar sign).

[snoyberg]

I changed my mind on this, and set up a useful error message as well. I think that should be sufficient.