Raise more helpful exception when concat() wrongly called as concat(obj1,obj2)
concat is supposed to be called with objs = a list or dict. Not multiple objects as as separate args. RIGHT: concat([df1,df2]) WRONG: concat(df1,df2)
The exception thrown in the latter case is seriously cryptic.

---

df1 = pd.DataFrame(randn(5,3), columns=['a','b','c'])
df2 = pd.DataFrame(randn(5,3), columns=['a','b','c'])
pd.concat(df1,df2)
... File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/frame.py", line 587, in **nonzero**
    raise ValueError("Cannot call bool() on DataFrame.")
## ValueError: Cannot call bool() on DataFrame.

Version: 0.10.1
