Need to check for empty InChI/InChI key and report

[cjh1]

We currently have a check for atom count. However, we also need to check if the convertion fails. For example:

$ python mongochem.py ferrocene.cml ferrocene
{'name': 'ferrocene', 'filename': 'ferrocene.cml'}
==============================
*** Open Babel Warning  in GetAtomicNum
  Cannot understand the element label X.
==============================
*** Open Babel Warning  in GetAtomicNum
  Cannot understand the element label X.
==============================
*** Open Babel Warning  in InChI code
  m1 :Unknown element(s): Xx
==============================
*** Open Babel Error  in InChI code
  InChI generation failed
==============================
*** Open Babel Warning  in InChI code
  m1 :Unknown element(s): Xx
==============================
*** Open Babel Error  in InChI code
  InChI generation failed
JSON output follows:
[{"inchikey": "", "xyz": "23\nm1\nC          3.60742        1.17022
    1.70118\nC          2.45209        1.79763        2.19417\nC
   1.36958        1.40787        1.38882\nC          1.85638
0.53814        0.39997\nC          3.23919        0.39222
0.59286\nC          2.15027       -0.67100        4.09665\nC
3.14638       -1.40054        3.42840\nC          2.53860
-2.09663        2.37108\nC          1.16761       -1.79507
2.38537\nC          0.92800       -0.91485        3.45213\nFe
2.24399       -0.15862        2.19981\nXx         2.50493
1.06122        1.25540\nXx         1.98617       -1.37562
3.14673\nH          4.61389        1.26971        2.10767\nH
2.40402        2.46920        3.05136\nH          0.33369
1.72469        1.50995\nH          1.26456        0.06005
-0.38059\nH          3.90960       -0.21830       -0.01205\nH
2.29969       -0.02792        4.96395\nH          4.20544
-1.42263        3.68532\nH          3.04244       -2.75525
1.66364\nH          0.42022       -2.17733        1.69011\nH
-0.03811       -0.49429        3.73118\n", "inchi": "", "name":
"ferrocene"}]