Fix LOCKFILE fallback

`throw` needs to provide an actual std::exception or it will not be
caught

(cherry picked from commit 844226e6b9)

include/lockfile.h

@@ -105,7 +105,7 @@ public:
             }
             else
             {
-                throw;
+                throw std::runtime_error( "Failed to open lock file" );
             }
         }
         catch( std::exception& e )