Confusing exception message when re-sampling index containing NaT
#### Example

```python
import pandas as pd
from pytz import UTC
pd.DataFrame([1,2], index=[pd.NaT, pd.datetime(2017, 5, 15, tzinfo=UTC)]).resample("W").sum()
```
#### Problem description

If a DataFrame's index contains `NaT`, when you try to resample  it you get the error: `ValueError: Passed item and index have different timezone`. This is confusing, because while technically correct, it leads the user to investigate timezones, and not the presence of missing times.

The error message lead me down a rabbit hole of trying to understand how a wrong timezone had crept in, but the timezones were consistent - it was the `NaT` which is a problem. I was not expecting my (large, >500k) index to contain `NaT`. There was a single `NaT` entry buried right in the middle of it and I had to bisect the dataset to figure out why it wasn't working.

[Searching for the error message shows up very few hits (currently 3, mostly source code).](https://duckduckgo.com/?q=%22Passed+item+and+index+have+different+timezone%22&t=canonical&ia=web)

#### Expected Output

Either to work and omit the value, or an error message which makes it clear that the problem is a `NaT` value, not a timezone issue.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-1013-aws
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 35.0.2
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 6.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.1.2
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: 0.0.9
pandas_gbq: None
pandas_datareader: None
</details>