Fix exception throwing

[yoshoku]

This pull request fixes exception throwing. For example, this pull request changes make the error messages appropriate.

Currently release version:

>>> import hnswlib
>>> p = hnswlib.Index(space='foo', dim=8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Caught an unknown exception!
>>> p = hnswlib.Index(space='l2', dim=8)
>>> p.init_index(max_elements=8)
>>> p.init_index(max_elements=8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Caught an unknown exception!
>>>

Applying this pull request changes:

>>> import hnswlib
>>> p = hnswlib.Index(space='foo', dim=8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Space name must be one of l2, ip, or cosine.
>>> p = hnswlib.Index(space='l2', dim=8)
>>> p.init_index(max_elements=8)
>>> p.init_index(max_elements=8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: The index is already initiated.
>>>

[yurymalkov]

Thank you!