check whether the search is a property in help function

If we call `math.help(x)` with an Object `x` which is not in `math`, we can get error message:
```
Error: No documentation found on "null"
```
If the property is not found, `text` remains `null`. Furthermore, in this error message, we can't determine whether the property is not in MathJS, or it is not documented (e.g. `math.eval("help(m)")` throws error but it is valid). Thus, I add an error message to handle this case.