ENH: use dictionary::readEntry for detection of input errors (#762, #1033)

- instead of   dict.lookup(name) >> val;
  can use      dict.readEntry(name, val);

  for checking of input token sizes.
  This helps catch certain types of input errors:

  {

      key1 ;                // <- Missing value
      key2 1234             // <- Missing ';' terminator
      key3 val;
  }

STYLE: readIfPresent() instead of 'if found ...' in a few more places.

src/rigidBodyMeshMotion/rigidBodyMeshMotion/rigidBodyMeshMotion.C

@@ -122,7 +122,7 @@ Foam::rigidBodyMeshMotion::rigidBodyMeshMotion
 {
     if (rhoName_ == "rhoInf")
     {
-        rhoInf_ = readScalar(coeffDict().lookup("rhoInf"));
+        readEntry("rhoInf", rhoInf_);
     }
 
     if (coeffDict().found("ramp"))