[plt-scheme] error in student languages

The situation with `error' is indeed in flux and confusing.

Here's a recap:

 Version 4.2.2 and before: `error' in ISL (and other HtDP languages)
   expected 2 arguments: symbol and string.

 Version 4.2.3: `error' changed to expect a single string argument.

 Version 4.2.3.x (SVN and nightly builds): `error' takes any number of
   arguments; it isn't currently documented correctly.