Keyword not found recommendations

For issue #888.
I promised no more comments about code but notice one more thing here. The above `if` block is pretty long. It could be easily shortened by changing condition to negative:

```python
if handler is not None:
    return handler
# error handling code here
```

or by moving the error handling code to a helper method:

```python
if hander is None:
    self._raise_no_keyword_found(name)
return handler
```

The latter would probably be cleaner.