`JsonNode.requiredAt()` does NOT fail on some path expressions

Looks like `JsonNode.requiredAt()` does not catch one type of missing references: those trying to traverse through non-container (scalar) nodes:

For example, for document:

```
{ "simple" : 5 }
```

should fail for

```
JsonNode n = mapper.readTree("{\"simple\":5}");
n.requiredAt("/simple/property")
```

but it does not.


