(python) Throw an exception if the index file can't be opened

[nickzoic]

This has caught me out a few times so I thought I'd make it a PR.

before:

>>> import mappy
>>> a = mappy.Aligner("notafile")
>>> a.seq_names
[]

no exception is thrown and it's not immediately obvious that the filename is incorrect.

after:

>>> import mappy
>>> a = mappy.Aligner("notafile")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "python/mappy.pyx", line 151, in mappy.Aligner.__cinit__
    PyErr_SetFromErrnoWithFilenameObject(OSError, fn_idx_in)
FileNotFoundError: [Errno 2] No such file or directory: 'notafile'

It can also throw PermissionError: [Errno 13] Permission denied: 'unreadablefile' and so on.