BUG: TimedeltaIndex raising ValueError when slice indexing
This is tangentially related to #14946.

#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df = pd.DataFrame({'x': range(10)})
df.index = pd.to_timedelta(df.index, unit='s')

# Each of these lines work as intended
df.loc[df.index[0], 'x'] = 10
df.loc[df['x'] > 1, 'x'] = 20
df.iloc[[0, 2], 0] = 30
df.iloc[2:4, 0] = 40

# These lines fail however
df.loc[df.index[7:9], 'x'] = 50
df.loc[df.index[3:5], 'x'] = 60
```

#### Problem description

This behavior is broken since 0.19.1 and is still occurring with the latest version of pandas. The problem is pretty self-explanatory.

**ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().**

#### Expected Output

The output is self-explanatory.

It can be easily fixed by adding the following to core/indexes/timedeltas.py TimedeltaIndex:

```python
def get_loc(self, key, method=None, tolerance=None):
    if is_bool_indexer(key) or is_timedelta64_dtype(key):
        raise TypeError
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-53-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8

pandas: 0.18.1
nose: None
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.2
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.3
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.4
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: None
boto: None
pandas_datareader: None

</details>
