stdlib: Change missing doc file to be a warning

When we cannot find the documentation file, we issue a warning
instead of error do that compilation can complete anyway. This
so that we don't have to have the docs available when compiling
a file.

lib/stdlib/src/epp.erl

@@ -1232,6 +1232,12 @@ scan_filedoc_content({string, _A, DocFilename}, Dot,
                     epp_reply(From, {error,{DocLoc,epp,{Doc, file, DocFilename}}}),
                     wait_req_scan(St)
             end;
+        {error, enoent} ->
+            epp_reply(From, {warning,{DocLoc,epp,{Doc, file, DocFilename}}}),
+            epp_reply(From, {ok,
+                             [{'-',DocLoc}, {atom, DocLoc, Doc}]
+                             ++ [{string, DocLoc, ""}, {dot,DocLoc}]}),
+            wait_req_scan(St);
         {error, _} ->
             epp_reply(From, {error,{DocLoc,epp,{Doc, file, DocFilename}}}),
             wait_req_scan(St)

lib/stdlib/test/epp_SUITE.erl

@@ -159,11 +159,11 @@ moduledoc_include(Config) when is_list(Config) ->
     {attribute, _, moduledoc, ModuleDoc} = lists:keyfind(moduledoc, 3, List),
     ?assertEqual({ok, unicode:characters_to_binary(ModuleDoc)}, file:read_file(DocName)),
 
-    %% negative test: checks that we produce an expected error
-    ModuleErrContent = binary:replace(ModuleFileContent, <<"README">>, <<"NotExistingFile">>),
-    ModuleErrName = CreateFile("module_attr", "moduledoc_err.erl", ModuleErrContent),
-    {ok, ListErr} = epp:parse_file(ModuleErrName, []),
-    {error,{_,epp,{moduledoc,file, "NotExistingFile.md"}}} = lists:keyfind(error, 1, ListErr),
+    %% negative test: checks that we produce an expected warning
+    ModuleWarnContent = binary:replace(ModuleFileContent, <<"README">>, <<"NotExistingFile">>),
+    ModuleWarnName = CreateFile("module_attr", "moduledoc_err.erl", ModuleWarnContent),
+    {ok, ListWarn} = epp:parse_file(ModuleWarnName, []),
+    {warning,{_,epp,{moduledoc,file, "NotExistingFile.md"}}} = lists:keyfind(warning, 1, ListWarn),
 
     ok.