Warn when a command is not implemented. Fail when no supplied commands are implemented

Rebar currently doesn't give any feedback on an invalid command. This change
makes rebar keep track of how many operations each command triggers, if a
particular command doesn't change the count, there were no modules implementing
it. If at the end of handling all commands, tje count is 0, none of the supplied
commands were valid and ?FAIL is called to trigger a non zero exit status.

src/rebar_core.erl

@@ -79,6 +79,9 @@ run(RawArgs) ->
     %% Note the top-level directory for reference
     rebar_config:set_global(base_dir, filename:absname(rebar_utils:get_cwd())),
 
+    %% Keep track of how many operations we do, so we can detect bad commands
+    erlang:put(operations, 0),
+
     %% Process each command, resetting any state between each one
     process_commands(CommandAtoms).
 
@@ -275,12 +278,26 @@ filter_flags([Item | Rest], Commands) ->
     end.
 
 process_commands([]) ->
-    ok;
+    case erlang:get(operations) of
+        0 ->
+            %% none of the commands had an effect
+            ?FAIL;
+        _ ->
+            ok
+    end;
 process_commands([Command | Rest]) ->
     %% Reset skip dirs
     [erlang:erase({skip_dir, D}) || D <- skip_dirs()],
+    Operations = erlang:get(operations),
 
     process_dir(rebar_utils:get_cwd(), rebar_config:new(), Command, sets:new()),
+    case erlang:get(operations) of
+        Operations ->
+            %% This command didn't do anything
+            ?CONSOLE("Command '~p' not understood\n", [Command]);
+        _ ->
+            ok
+    end,
     process_commands(Rest).
 
 
@@ -417,6 +434,9 @@ execute(Command, Modules, Config, ModuleFile) ->
             Dir = rebar_utils:get_cwd(),
             ?CONSOLE("==> ~s (~s)\n", [filename:basename(Dir), Command]),
 
+            %% Increment the count of operations, since some module responds to this command
+            erlang:put(operations, erlang:get(operations) + 1),
+
             %% Run the available modules
             case catch(run_modules(TargetModules, Command, Config, ModuleFile)) of
                 ok ->