BUG: DataFrame.drop can't find row label when iterating through dict
When using a for loop with a dictionary of dataframes, DataFrame.drop() raises an unexpected error, apparently unable to find the label, even when that label exists and is handled correctly in a single df.
#### A small, complete example of the issue

``` python
>>> for key in dfs2:
>>>    dfs2[key].drop('0',axis=0,inplace=True)
---------------------------------------------------
ValueError        Traceback (most recent call last)
<ipython-input-179-3b991fac4b08> in <module>()
      1 for key in dfs2:
----> 2     dfs2[key].drop('0',axis=0,inplace=True)

/Library/Python/2.7/site-packages/pandas/core/generic.pyc in drop(self, labels, axis, level, inplace, errors)
   1875                 new_axis = axis.drop(labels, level=level, errors=errors)
   1876             else:
-> 1877                 new_axis = axis.drop(labels, errors=errors)
   1878             dropped = self.reindex(**{axis_name: new_axis})
   1879             try:

/Library/Python/2.7/site-packages/pandas/indexes/base.pyc in drop(self, labels, errors)
   3049             if errors != 'ignore':
   3050                 raise ValueError('labels %s not contained in axis' %
-> 3051                                  labels[mask])
   3052             indexer = indexer[~mask]
   3053         return self.delete(indexer)

ValueError: labels ['0'] not contained in axis

```

Iterating through a dict with DataFrame.drop _does_ behave as expected (viz. successfully drop the row) when using `errors='ignore'`.  However, I expected iteration with drop to behave as drop does with a single dataframe from the same dict:
#### Expected Output

``` python
>>> dfs2[key]
   A  B  C  D
a  1  2  3  4
b  2  3  4  5
0  3  4  5  6
d  4  5  6  7
e  5  6  7  8
>>> dfs2[key].drop('0',axis=0,inplace=True)
>>> dfs2[key]
   A  B  C  D
a  1  2  3  4
b  2  3  4  5
d  4  5  6  7
e  5  6  7  8
```

If different behavior for .drop when iterating is intended, a note in the docs might be helpful.
#### Output of `pd.show_versions()`

<details>
## INSTALLED VERSIONS

commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 15.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 1.1.6
Cython: None
numpy: 1.8.0rc1
scipy: 0.13.0b1
statsmodels: None
xarray: None
IPython: 5.0.0
sphinx: None
patsy: None
dateutil: 1.5
pytz: 2013.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.3.1
openpyxl: 2.3.5
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.9.3
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: None

</details>
