API: Poor error message on colspec validation.
AssertionError bubbling up to the user - ruroh:

``` python
from io import StringIO
data = StringIO(u"""
121301234
121300123
121300012
""")
pd.read_fwf(data, colspecs=[4, 5])
```

Ends up with an AssertionError:

```
/opt/anaconda/envs/np17py27-1.5/lib/python2.7/site-packages/pandas/io/parsers.pyc in __init__(self, f, colspecs, filler, thousands)
   1771         assert isinstance(colspecs, (tuple, list))
   1772         for colspec in colspecs:
-> 1773             assert isinstance(colspec, (tuple, list))
   1774             assert len(colspec) == 2
   1775             assert isinstance(colspec[0], int)

AssertionError: 
```

Low priority but we should fix at some point.  Really could be as simple as this in `read_fwf`. `if not all(isinstance(colspec, (tuple, list) and len(colspec) == 2 and isinstance(colspec[0], int) for colspec in colspecs): raise ValueError("colspecs must be 2-tuples of integers")`
