catch error on multiplpe app files in an app dir

src/rebar_app_discover.erl

@@ -70,7 +70,9 @@ find_app(AppDir, Validate) ->
                            [F] ->
                                rebar_app_info:app_file_src(AppInfo1, F);
                            [] ->
-                               AppInfo1
+                               AppInfo1;
+                           Other when is_list(Other) ->
+                               throw({error, {multiple_app_files, Other}})
                        end,
             case Validate of
                 valid ->
@@ -101,8 +103,12 @@ find_app(AppDir, Validate) ->
                             false
                     end;
                 [] ->
-                    false
-            end
+                    false;
+                Other when is_list(Other) ->
+                    throw({error, {multiple_app_files, Other}})
+            end;
+        Other when is_list(Other) ->
+            throw({error, {multiple_app_files, Other}})
     end.
 
 app_dir(AppFile) ->

src/rebar_prv_app_discovery.erl

@@ -33,9 +33,16 @@ init(State) ->
 -spec do(rebar_state:t()) -> {ok, rebar_state:t()} | {error, string()}.
 do(State) ->
     LibDirs = rebar_state:get(State, lib_dirs, ?DEFAULT_LIB_DIRS),
-    State1 = rebar_app_discover:do(State, LibDirs),
-    {ok, State1}.
-
--spec format_error(any()) ->  iolist().
+    try
+        State1 = rebar_app_discover:do(State, LibDirs),
+        {ok, State1}
+    catch
+        throw:{error, Error}->
+            {error, {?MODULE, Error}}
+    end.
+
+-spec format_error(any()) -> iolist().
+format_error({multiple_app_files, Files}) ->
+    io_lib:format("Multiple app files found in one app dir: ~s", [string:join(Files, " and ")]);
 format_error(Reason) ->
     io_lib:format("~p", [Reason]).