confusing ImportError message in `__init__.py`
In some cases the error message could be more clear, as discussed here:
https://github.com/pydata/pandas/commit/69cab57c9889391a587656ff8c6a7f9b38187bb1#commitcomment-15762471

my example:

```
>>> import pandas
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/marcin/install/lib/python2.7/site-packages/pandas/__init__.py", line 13, in <module>
    "extensions first.".format(module))
ImportError: C extension: No module named pytz.tzinfo not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace' to build the C extensions first.
```
