clean up cli

internal/engine.cm

@@ -728,7 +728,7 @@ function report_to_overling(msg)
 var program = _cell.args.program
 
 if (!program) {
-  log.error('No program specified. Usage: cell <program.ce> [args...]')
+  log.error('No program specified. Usage: cell <program> [args...]')
   os.exit(1)
 }
 
@@ -819,7 +819,7 @@ actor_mod.setname(_cell.args.program)
 
 var prog = _cell.args.program
 if (ends_with(prog, '.cm')) {
-  os.print(`error: ${prog} is a module (.cm), not an actor (.ce). Run it with: cell --core <path> ${prog}\n`)
+  os.print(`error: ${prog} is a module (.cm), not a program (.ce)\n`)
   os.exit(1)
 }
 if (ends_with(prog, '.ce')) prog = text(prog, 0, -3)
@@ -868,6 +868,8 @@ $_.clock(_ => {
   env.use = function(path) {
     var ck = 'core/' + path
     if (use_cache[ck]) return use_cache[ck]
+    var core_mod = use_core(path)
+    if (core_mod) return core_mod
     return shop.use(path, pkg)
   }
   env.args = _cell.args.arg