How to use loc WITHOUT enlargement in pandas >0.14
It appears that `loc` now enlarges the result if an index is not found  - a behavior I find confusing, because 

``` python
df.loc[['missing_label']]
```

gives a `KeyError` while 

``` python
df.loc[['existing_label', 'missing_label']]
```

works just fine. Apparently this was changed in 0.15. I have older code that relied on catching errors for the case where some labels might be missing, which now doesn't work. How can I do a `loc` select, guaranteeing that I will get an error if a non-existing label is given - i.e. do the same as `loc` but without ever enlarging the result?
