Turn REST response exceptions into `GoogleAPICallError`

[busunkim96]

Currently the REST transport uses response.raise_for_status() and returns the exception object from requests.

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest.py.j2

Line 206 in cc55a18

response.raise_for_status()

For consistency with existing libraries the response should be converted into a GoogleAPICallError.

https://github.com/googleapis/python-api-core/blob/db48ea3f8f69cd5039cdc2e35a0a3604d731bc70/google/api_core/exceptions.py#L399

[yon-mg]

Addressed in upcoming pr.