Fix 1 occurrence of always-throwing-if-to-when

Using `when` and `unless` is simpler than a conditional with an always-throwing branch.

Author: resyntax-ci[bot]

scribble-lib/scriblib/gui-eval.rkt

@@ -101,9 +101,9 @@
      (λ (gui-eval get-predicate? get-render get-get-width get-get-height)
        (lambda (ev catching-exns? expr)
          (with-handlers ([exn:fail? (lambda (exn)
-                                      (if catching-exns?
-                                          (raise exn)
-                                          (void)))])
+                                      (when catching-exns?
+                                        (raise exn))
+                                      (void))])
            (define v (read log-file))
            (if (eof-object? v)
                (error "expression not in log file")