Static maps fail silently and produces malformed data

Please be sure to include as much information as possible:

#### Environment details

1. Specify the API at the beginning of the title (for example, "Places: ..."): static maps
2. OS type and version
3. Library version and other environment information: latest master

#### Steps to reproduce

1. Request static maps that fail (e.g. wrong API key)
2. Iterate the response

#### Code example

```python
client = googlemaps.Client(key="asd")
response = client.static_map(
size=(640, 640),
zoom=zoom,
center=center,
maptype="satellite",
format="png",
scale=1,
)

for r in response:
print(r)
```

#### Output
```
b'T' b'he ' b'G' b'oo' b'gl' b'e ' b'M' b'ap' b's ' b'P' b'la' b't' b'fo' b'rm' b' s' b'er' b've' b'r ' b're' b'je' b'ct' b'ed' b' ' b'yo' b'u' b'r re' b'q' b'ue' b'st' b'. ' b'T' b'he ' b'pr' b'ov' b'i' b'd' b'ed ' b'AP' b'I ' b'k' b'ey' b' i' b's ' b'i' b'nv' b'a' b'li' b'd.' b' '
```

Since the static maps function passes in it's own `extract_body` function to `_request`, the error checking doesn't happen.
See [here](https://github.com/googlemaps/google-maps-services-python/blob/a93b3c0fe1ec4680fbb34fc29ac8714b9a8c7da4/googlemaps/client.py#L312).

Possible solution: #446