signal_catcher: print errno when tombstoned_notify_completion fails.

Failure here could either be someone internal closing our FD (EBADF) or
tombstoned closing its end of the socket (EPIPE). Print the error to
disambiguate the cases.

Bug: 67805174
Test: treehugger
Change-Id: Ie35cb31218b5bb08757e61360c6539d39eece08b
diff --git a/runtime/signal_catcher.cc b/runtime/signal_catcher.cc
index b50879f..a1f14be 100644
--- a/runtime/signal_catcher.cc
+++ b/runtime/signal_catcher.cc
@@ -170,7 +170,7 @@
 
 #if defined(ART_TARGET_ANDROID)
   if (use_tombstoned_stack_trace_fd_ && !tombstoned_notify_completion(tombstone_fd)) {
-    LOG(WARNING) << "Unable to notify tombstoned of dump completion.";
+    PLOG(WARNING) << "Unable to notify tombstoned of dump completion";
   }
 #endif
 }