Catch errors from invalid macro while expanding

[JesperAxelsson]

Fix Lem UI deadlock by catching compiler errors in macrostep-expand-1

When expanding a syntactically malformed macro, macrostep-expand-1 fails
to catch low-level compiler errors. These uncaught errors bypass the local
error-message check and trigger the interactive Swank/Micros debugger.

Because the Lem UI thread is blocked synchronously waiting for an
(:ok ...) or (:error ...) response list, the editor deadlocks.

This wraps the expansion step in macrostep-expand-1 with a handler-case
ensuring fatal compiler crashes are safely caught
and returned as standard (:error ...) lists.

How to recreate in lem

If you have a broken macro like this:

(defmacro borked-macro (a b)
  (`(+ ,a ,b)))

And try to expand it in a buffer with lisp-macrostep-expand or C-c enter.

(borked-macro 1 2)

Let me know if there anything you want to change/add.