Make diameter_app_SUITE fail more usefully

The badmatch in the grandparent commit is ssl not being installed:

=== Reason: no match of right hand side value {ssl,non_existing,
                                                    ["non_existing"]}
  in function  diameter_app_SUITE:app/1 (diameter_app_SUITE.erl, line 266)
  in call from diameter_app_SUITE:'-xref/1-fun-3-'/2 (diameter_app_SUITE.erl, line 214)
  in call from lists:'-filter/2-lc$^0/1-0-'/2 (lists.erl, line 1286)
  in call from diameter_app_SUITE:xref/1 (diameter_app_SUITE.erl, line 214)
  in call from test_server:ts_tc/3 (test_server.erl, line 1545)
  in call from test_server:run_test_case_eval1/6 (test_server.erl, line 1063)
  in call from test_server:run_test_case_eval/9 (test_server.erl, line 995)

Tweak the failure a bit more, without fixing it.

1 files changed, 5 insertions, 3 deletions

diff --git a/lib/diameter/test/diameter_app_SUITE.erl b/lib/diameter/test/diameter_app_SUITE.erl
index 3c1313c296..ef4a28d3f4 100644
--- a/lib/diameter/test/diameter_app_SUITE.erl
+++ b/lib/diameter/test/diameter_app_SUITE.erl
@@ -208,9 +208,9 @@ xref(Config) ->
     CTmods = CTmods -- Mods,
 
     %% Ensure that runtime modules only call other runtime modules, or
-    %% applications declared as in runtime_dependencies in the app
-    %% file. Note that the declared application versions are ignored
-    %% since we only know what we can see now.
+    %% applications declared in runtime_dependencies in the app file.
+    %% The declared application versions are ignored since we only
+    %% know what we see now.
     [] = lists:filter(fun(M) -> not lists:member(app(M), Deps) end,
                       RTdeps -- Mods).
 
@@ -261,6 +261,8 @@ app(Mod) ->
     case code:which(Mod) of
         preloaded ->
             "erts";
+        Reason when is_atom(Reason) ->
+            error({Reason, Mod});
         Path ->
             %% match to identify an unexpectedly short path
             {_, _, [_,_,_|_] = Split} = {Mod, Path, filename:split(Path)},