raco demod: report that it works only on BC

Closes #3706

pkgs/compiler-lib/compiler/demodularizer/batch.rkt

@@ -16,7 +16,10 @@
                  (garbage-collect-toplevels-enabled #t)]                         
                 [("-r" "--recompile") "Recompile final module to re-run optimizations"
                  (recompile-enabled #t)]                         
-                #:args (filename) 
+                #:args (filename)
+                (unless (eq? 'racket (system-type 'vm))
+                  (raise-user-error (format "~a: supported only on Racket BC"
+                                            (short-program+command-name))))
                 (demodularize filename (output-file))))
 
 (module test racket/base)

pkgs/racket-doc/scribblings/raco/demod.scrbl

@@ -3,10 +3,11 @@
 
 @title[#:tag "demod"]{@exec{raco demod}: Demodularizing Programs}
 
-The @exec{raco demod} command takes a Racket module and flattens
-all of its dependencies into a single compiled module. 
-A file @filepath{@nonterm{name}.rkt} is demodularized into 
+The @exec{raco demod} command, which works only for Racket @tech[#:doc
+guide-doc]{BC}, takes a Racket module and flattens all of its
+dependencies into a single compiled module. A file
+@filepath{@nonterm{name}.rkt} is demodularized into
 @filepath{@nonterm{name}_rkt_merged.zo}.
 
-The demodularized zo file can be run by passing it as an argument to
-the @exec{racket} command-line program.
+The demodularized @filepath{.zo} file can be run by passing it as an
+argument to the @exec{racket} command-line program.