ENH: add errors='raise' option to rename
Is there a reason that `.rename` doesn't raise if keys are supplied that don't exist? This caught me out a couple of times:

``` python
In [14]: import pandas as pd

In [15]: df=pd.DataFrame({'a': [1,2], 'b': [3,4]})

In [16]: df
Out[16]: 
   a  b
0  1  3
1  2  4

In [17]: df.rename(columns={'a': 'c'})
Out[17]: 
   c  b
0  1  3
1  2  4
# ok

In [18]: df.rename(columns={'d': 'c'})
Out[18]: 
   a  b
0  1  3
1  2  4

# this should raise?
```
