More Gambit-y errors from the macro expander

[per-gron]

Right now, macro expansion errors are reported with the error procedure. For instance, (cond-expand a) generates the error *** ERROR IN ##main -- Invalid cond-expand form: (cond-expand a), but this is not optimal, since it doesn't give information about the source location. In Gambit there is a procedure called c#compiler-user-error which is used internally to generate Gambit's error messages. It might be better to use that instead of just error.

I'm not entirely sure that this is a desirable thing, but I think so. Fixing this would mean to implement a small utility function that calls c#compiler-user-error and put it in util.scm, and then searching through the calls to error in hygiene.scm, replacing the calls where appropriate.

[alvatar]

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