[yaml2obj] Improve error message when output file cannot be opened

[davide]

Can you add a testcase to show the difference?

[sbc100]

• add test
• Merge remote-tracking branch 'origin/master' into yaml2obj_error
• update expected output

[sbc100]

While working on this I noticed that the other tests in test/yaml2obj/ don't even run because they have the wrong extension. I upload a separate change for that

[sbc100]

This change should wait until D31436 lands

[sbc100]

Could you land this for me? (I'm not yet a committer)

[davide]

Sure, I'll commit this in a sec.

tools/yaml2obj/yaml2obj.cpp

int main(int argc, char **argv) {
 
   if (OutputFilename.empty())
     OutputFilename = "-";
 
   std::error_code EC;
   std::unique_ptr<tool_output_file> Out(
       new tool_output_file(OutputFilename, EC, sys::fs::F_None));
   if (EC) {
-    errs() << EC.message() << '\n';
+    errs() << "Error: '" << EC.message() << "'\n";
     return 1;
   }
 
   ErrorOr<std::unique_ptr<MemoryBuffer>> Buf =
       MemoryBuffer::getFileOrSTDIN(Input);
   if (!Buf)
     return 1;
 
   yaml::Input YIn(Buf.get()->getBuffer());
 
   int Res = convertYAML(YIn, Out->os());
   if (Res == 0)
     Out->keep();
 
   return Res;
 }