.loc[list-of-float-labels] yields new rows instead of KeyError
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame(
    {
        "a":[11, 11, 22],
        "i":[1081., 1071., 22.],
    },
)
df = df.set_index("i")
# df.index = df.set_index("i").astype(int)
print df.loc[[-99999999]]
```
#### Problem description

The above gets:
```
            a
i            
-99999999 NaN
```
... where did this row come from? Was not in my data.

#### Expected Output

```KeyError: u'None of [[-99999999]] are in the [index]'``

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-24-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 36.0.1
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.3.0
sphinx: None
patsy: 0.2.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 1.7.0
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
