Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/monitor.cc b/runtime/monitor.cc
index 6123934..5020ced 100644
--- a/runtime/monitor.cc
+++ b/runtime/monitor.cc
@@ -877,7 +877,7 @@
     }
     default: {
       LOG(FATAL) << "Unreachable";
-      return ThreadList::kInvalidThreadId;
+      UNREACHABLE();
     }
   }
 }
@@ -1032,7 +1032,7 @@
       return true;
     default:
       LOG(FATAL) << "Unreachable";
-      return false;
+      UNREACHABLE();
   }
 }