Can set but not get item with tuple index
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

set1 = ['a', 'b', 'c']
set2 = ['1', '2', '3']
set3 = ['test']

tuple_index = [(s1, s2) for s1 in set1 for s2 in set2]
df = pd.DataFrame(index=tuple_index, columns=set3)

df.loc[('a', '1'), 'test'] = 5
print(df)  # Shows assignment occurs
print(df.loc[('a', '1'), 'test'])  # Raises KeyError: "None of [('a', '1')] are in the [index]"

```
#### Problem description

Indexing with tuples is permitted only for setting but not getting.

#### Expected Output

Should either:
1. Refuse to set the item because using tuple indicies is a silly idea, see #20991
2. Successfully get the item.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.22.0
pytest: 3.2.3
pip: 10.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.3.1
sphinx: None
patsy: 0.5.0
dateutil: 2.7.0
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
Credit @jameshowse for finding.