Make use of exit status to detect missing module

src/shellfu.sh.skel

@@ -132,8 +132,7 @@ shellfu() {
             local es=4
             local mpath
             shellfu __debug "importing module $mname"
-            mpath=$(shellfu _select_mfile "$mname")
-            test -n "$mpath" || {
+            mpath=$(shellfu _select_mfile "$mname") || {
                 shellfu __warn "cannot find module: $mname"
                 return 3
             }
@@ -180,7 +179,7 @@ shellfu() {
             #
             # Select file that contains module named $1
             #
-            shellfu _list_mfiles "$1" | head -1
+            shellfu _list_mfiles "$1" | grep -m1 .
             ;;
 
         *)