 Wrong "Too many indexers" error asking for missing key in Series MultiIndex
#### Code Sample, a copy-pastable example if possible

```python
In [2]: s = pd.Series(-1, index=pd.MultiIndex.from_product([[0, 1]]*2))

In [3]: s.loc[0, 3]
---------------------------------------------------------------------------
IndexingError                             Traceback (most recent call last)
<ipython-input-3-16ed32312931> in <module>()
----> 1 s.loc[0, 3]

/home/nobackup/repo/pandas/pandas/core/indexing.py in __getitem__(self, key)
   1470             except (KeyError, IndexError):
   1471                 pass
-> 1472             return self._getitem_tuple(key)
   1473         else:
   1474             # we by definition only have the 0th axis

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_tuple(self, tup)
    873 
    874         # no multi-index, so validate all of the indexers
--> 875         self._has_valid_tuple(tup)
    876 
    877         # ugly hack for GH #836

/home/nobackup/repo/pandas/pandas/core/indexing.py in _has_valid_tuple(self, key)
    218         for i, k in enumerate(key):
    219             if i >= self.obj.ndim:
--> 220                 raise IndexingError('Too many indexers')
    221             try:
    222                 self._validate_key(k, i)

IndexingError: Too many indexers

In [4]: s.loc[3, 1]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/home/nobackup/repo/pandas/pandas/core/indexing.py in _validate_key(self, key, axis)
   1789                 if not ax.contains(key):
-> 1790                     error()
   1791             except TypeError as e:

/home/nobackup/repo/pandas/pandas/core/indexing.py in error()
   1784                                .format(key=key,
-> 1785                                        axis=self.obj._get_axis_name(axis)))
   1786 

KeyError: 'the label [3] is not in the [index]'

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-4-0ff3baa1fa82> in <module>()
----> 1 s.loc[3, 1]

/home/nobackup/repo/pandas/pandas/core/indexing.py in __getitem__(self, key)
   1470             except (KeyError, IndexError):
   1471                 pass
-> 1472             return self._getitem_tuple(key)
   1473         else:
   1474             # we by definition only have the 0th axis

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_tuple(self, tup)
    873 
    874         # no multi-index, so validate all of the indexers
--> 875         self._has_valid_tuple(tup)
    876 
    877         # ugly hack for GH #836

/home/nobackup/repo/pandas/pandas/core/indexing.py in _has_valid_tuple(self, key)
    220                 raise IndexingError('Too many indexers')
    221             try:
--> 222                 self._validate_key(k, i)
    223             except ValueError:
    224                 raise ValueError("Location based indexing can only have "

/home/nobackup/repo/pandas/pandas/core/indexing.py in _validate_key(self, key, axis)
   1796                 raise
   1797             except:
-> 1798                 error()
   1799 
   1800     def _is_scalar_access(self, key):

/home/nobackup/repo/pandas/pandas/core/indexing.py in error()
   1783                 raise KeyError(u"the label [{key}] is not in the [{axis}]"
   1784                                .format(key=key,
-> 1785                                        axis=self.obj._get_axis_name(axis)))
   1786 
   1787             try:

KeyError: 'the label [3] is not in the [index]'

```

#### Problem description

The second error is correct, the first is wrong. Might be related to #20951, which however is easier to explain given that it happens on a DataFrame, where there is the axis/level ambiguity.

#### Expected Output

``KeyError: 'the label [3] is not in the [index]'``

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

<details>

INSTALLED VERSIONS
------------------
commit: 1288e5269e576bb299be4d1cdd8062ec87a13470
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: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+23.g1288e5269
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.0.1
Cython: 0.25.2
numpy: 1.14.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.0dev
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: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1

</details>
