Downgrade missing library to a warning

Some RMW's like rmw_cyclonedds_cpp and rmw_fastrtps_dynamic_cpp (?) do not need to inject build dependencies.

Signed-off-by: Dan Rose <dan@digilabs.io>

rmw_implementation/rmw_implementation-extras.cmake.in

@@ -39,11 +39,14 @@ else()
   get_available_rmw_implementations(available_rmw_implementations)
 
   if(NOT requested_rmw_implementation IN_LIST available_rmw_implementations)
-    message(FATAL_ERROR
+    message(WARNING
       "The RMW implementation has been specified as "
       "'${requested_rmw_implementation}' via ${requested_rmw_implementation_from}, "
-      "however it is not in the list of supported rmw implementations, "
-      "'${available_rmw_implementations}'")
+      "but it is not available at this time.\n"
+      "Make sure it does not expect a typesupport or other build dependencies.\n\n"
+      "Currently available middlewares:"
+      "'${available_rmw_implementations}' "
+      )
   endif()
 
   set(selected_rmw_implementation "@PROJECT_NAME@")