Improve import_from_dotted_path error message

* Avoid exposing `rsplit` implementation detail in exception message.
* Fix #28.
```python
>>> try:
...  1/0
... except ZeroDivisionError:
...  raise TypeError
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
TypeError

>>> try:
...  1/0
... except ZeroDivisionError:
...  raise TypeError from None
... 
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
TypeError
```