Print a warning if the "positions" file is missing.

src/lagrangian/basic/Cloud/CloudIO.C

@@ -46,6 +46,13 @@ void Foam::Cloud<ParticleType>::initCloud(const bool checkClass)
             readFields();
         }
     }
+    else
+    {
+        WarningIn("Cloud<ParticleType>::initCloud(const bool checkClass)")
+            << "Cannot read particle positions file " << nl
+            << "    " << ioP.path() << nl
+            << "    assuming the initial cloud contains 0 particles." << endl;
+    }
 }