Exception while MultiIndex slice contains "min", "mean" or "max"
I want to slice the dataframe contains MultiIndex

```python
import pandas as pd
stages = ("a", "b", "c")
stats = ("s0", "s1", "s2")
index = pd.MultiIndex.from_product([stages, stats], names=("lev0", "lev1"))
df = pd.DataFrame(index=index, columns=["f0", "f1"])
df
```

![](https://user-images.githubusercontent.com/2182222/58710365-92762600-83d5-11e9-87c0-68013814a280.png)

Let's slicing

```python
df.loc[("b", "s2"):("c", "s1")]
```

![](https://user-images.githubusercontent.com/2182222/58710436-c3565b00-83d5-11e9-943d-f0869260a14b.png)

Let's change the ``stats`` value and recreate the dataframe:

```python
stats = ("min", "mean", "max")
index = pd.MultiIndex.from_product([stages, stats], names=("lev0", "lev1"))
df = pd.DataFrame(index=index, columns=["f0", "f1"])
df
```

![](https://user-images.githubusercontent.com/2182222/58710516-ea149180-83d5-11e9-9dc7-6e262d34e2cc.png)

and the similar slicing

```python
df.loc[("b", "max"):("c", "mean")]
```

raises ``UnsortedIndexError``:

<details>

```
---------------------------------------------------------------------------
UnsortedIndexError                        Traceback (most recent call last)
<ipython-input-68-21fd12d039a9> in <module>
----> 1 df.loc[("b", "max"):("c", "mean")]

c:\opt\miniconda3\lib\site-packages\pandas\core\indexing.py in __getitem__(self, key)
   1498 
   1499             maybe_callable = com.apply_if_callable(key, self.obj)
-> 1500             return self._getitem_axis(maybe_callable, axis=axis)
   1501 
   1502     def _is_scalar_access(self, key):

c:\opt\miniconda3\lib\site-packages\pandas\core\indexing.py in _getitem_axis(self, key, axis)
   1865         if isinstance(key, slice):
   1866             self._validate_key(key, axis)
-> 1867             return self._get_slice_axis(key, axis=axis)
   1868         elif com.is_bool_indexer(key):
   1869             return self._getbool_axis(key, axis=axis)

c:\opt\miniconda3\lib\site-packages\pandas\core\indexing.py in _get_slice_axis(self, slice_obj, axis)
   1531         labels = obj._get_axis(axis)
   1532         indexer = labels.slice_indexer(slice_obj.start, slice_obj.stop,
-> 1533                                        slice_obj.step, kind=self.name)
   1534 
   1535         if isinstance(indexer, slice):

c:\opt\miniconda3\lib\site-packages\pandas\core\indexes\base.py in slice_indexer(self, start, end, step, kind)
   4671         """
   4672         start_slice, end_slice = self.slice_locs(start, end, step=step,
-> 4673                                                  kind=kind)
   4674 
   4675         # return a slice

c:\opt\miniconda3\lib\site-packages\pandas\core\indexes\multi.py in slice_locs(self, start, end, step, kind)
   2303         # This function adds nothing to its parent implementation (the magic
   2304         # happens in get_slice_bound method), but it adds meaningful doc.
-> 2305         return super(MultiIndex, self).slice_locs(start, end, step, kind=kind)
   2306 
   2307     def _partial_tup_index(self, tup, side='left'):

c:\opt\miniconda3\lib\site-packages\pandas\core\indexes\base.py in slice_locs(self, start, end, step, kind)
   4870         start_slice = None
   4871         if start is not None:
-> 4872             start_slice = self.get_slice_bound(start, 'left', kind)
   4873         if start_slice is None:
   4874             start_slice = 0

c:\opt\miniconda3\lib\site-packages\pandas\core\indexes\multi.py in get_slice_bound(self, label, side, kind)
   2247         if not isinstance(label, tuple):
   2248             label = label,
-> 2249         return self._partial_tup_index(label, side=side)
   2250 
   2251     def slice_locs(self, start=None, end=None, step=None, kind=None):

c:\opt\miniconda3\lib\site-packages\pandas\core\indexes\multi.py in _partial_tup_index(self, tup, side)
   2310                 'Key length (%d) was greater than MultiIndex'
   2311                 ' lexsort depth (%d)' %
-> 2312                 (len(tup), self.lexsort_depth))
   2313 
   2314         n = len(tup)

UnsortedIndexError: 'Key length (2) was greater than MultiIndex lexsort depth (1)'
```
</details>

My environment:

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.24.2
pytest: 4.4.1
pip: 19.1
setuptools: 41.0.0
Cython: 0.29.7
numpy: 1.16.3
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.5.0
sphinx: 2.0.1
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.3
openpyxl: 2.6.2
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: None
lxml.etree: None
bs4: 4.7.1
html5lib: None
sqlalchemy: 1.3.3
pymysql: None
psycopg2: 2.7.6.1 (dt dec pq3 ext lo64)
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
```
​
</details>