No KeyError in partial indexing of unused label if placeholder is used in (only) some columns
#### Code Sample, a copy-pastable example if possible

```python
In [2]: mi = pd.MultiIndex(levels=[['a_lot', 'onlyone', 'notevenone'], [1970, '']],
   ...:            labels=[[1, 0], [1, 0]])
   ...: df = pd.DataFrame(-1, index=range(3), columns=mi)
   ...:            

In [3]: df
Out[3]: 
  onlyone a_lot
           1970
0      -1    -1
1      -1    -1
2      -1    -1

In [4]: df['notevenone']
Out[4]: 
Empty DataFrame
Columns: []
Index: [0, 1, 2]

```
#### Problem description

As in #19027 (but apparently a bit more subtle), a ``KeyError`` should be raised, as the presence of ``notevenone`` in the first level is an implementation detail.

#### Expected Output

``` python
In [5]: df['reallynot']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/home/pietro/nobackup/repo/pandas/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   3011             try:
-> 3012                 return self._engine.get_loc(key)
   3013             except KeyError:

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5729)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5575)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:22215)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:22169)()

KeyError: 'reallynot'

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-5-845e63863b1b> in <module>()
----> 1 df['reallynot']

/home/pietro/nobackup/repo/pandas/pandas/core/frame.py in __getitem__(self, key)
   2511             return self._getitem_frame(key)
   2512         elif is_mi_columns:
-> 2513             return self._getitem_multilevel(key)
   2514         else:
   2515             return self._getitem_column(key)

/home/pietro/nobackup/repo/pandas/pandas/core/frame.py in _getitem_multilevel(self, key)
   2555 
   2556     def _getitem_multilevel(self, key):
-> 2557         loc = self.columns.get_loc(key)
   2558         if isinstance(loc, (slice, Series, np.ndarray, Index)):
   2559             new_columns = self.columns[loc]

/home/pietro/nobackup/repo/pandas/pandas/core/indexes/multi.py in get_loc(self, key, method)
   2227 
   2228         if not isinstance(key, tuple):
-> 2229             loc = self._get_level_indexer(key, level=0)
   2230 
   2231             # _get_level_indexer returns an empty slice if the key has

/home/pietro/nobackup/repo/pandas/pandas/core/indexes/multi.py in _get_level_indexer(self, key, level, indexer)
   2486         else:
   2487 
-> 2488             loc = level_index.get_loc(key)
   2489             if isinstance(loc, slice):
   2490                 return loc

/home/pietro/nobackup/repo/pandas/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   3012                 return self._engine.get_loc(key)
   3013             except KeyError:
-> 3014                 return self._engine.get_loc(self._maybe_cast_indexer(key))
   3015 
   3016         indexer = self.get_indexer([key], method=method, tolerance=tolerance)

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5729)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5575)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:22215)()

/home/pietro/nobackup/repo/pandas/pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:22169)()

KeyError: 'reallynot'

```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.0.dev0+653.g7273ea070
pytest: 3.0.6
pip: 9.0.1
setuptools: 33.1.1
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.18.1
pyarrow: None
xarray: None
IPython: 5.2.2
sphinx: None
patsy: 0.4.1+dev
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: None
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None


</details>
