BigQuery: fix swallowed error message

[mistercrunch]

When getting a Cannot query over table (...) without a filter over column(s) (...), the re-raising happening gobbles up the error message
and the context is lost.

This may or may not be the right fix, but clearly highlights the issue
of self._query_job.errors being None in that particular context.
When that is the case, the only context available is that of
DatabaseError without any details which clearly isn't enough detail.

After this fix is applied (wrapping the exception into another exception) both stack traces are shown:
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/bigquery/job.py", line 2685, in result
    super(QueryJob, self).result(timeout=timeout)
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/bigquery/job.py", line 697, in result
    return super(_AsyncJob, self).result(timeout=timeout)
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/api_core/future/polling.py", line 115, in result
    self._blocking_poll(timeout=timeout)
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/bigquery/job.py", line 2659, in _blocking_poll
    super(QueryJob, self)._blocking_poll(timeout=timeout)
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/api_core/future/polling.py", line 94, in _blocking_poll
    retry_(self._done_or_raise)()
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/api_core/retry.py", line 260, in retry_wrapped_func
    on_error=on_error,
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/api_core/retry.py", line 177, in retry_target
    return target()
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/api_core/future/polling.py", line 73, in _done_or_raise
    if not self.done():
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/bigquery/job.py", line 2647, in done
    location=self.location)
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/bigquery/client.py", line 584, in _get_query_results
    retry, method='GET', path=path, query_params=extra_params)
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/bigquery/client.py", line 336, in _call_api
    return call()
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/api_core/retry.py", line 260, in retry_wrapped_func
    on_error=on_error,
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/api_core/retry.py", line 177, in retry_target
    return target()
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/_http.py", line 293, in api_request
    raise exceptions.from_http_response(response)
google.api_core.exceptions.BadRequest: 400 GET https://www.googleapis.com/bigquery/v2/projects/fsql-production/queries/6ed337a4-e23c-4634-ba94-15254648bdfb?maxResults=0&location=US: Cannot query over table 'fsql-production.records.routes_stream' without a filter over column(s) 'created_at' that can be used for partition elimination

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mbeauchemin/incubator-superset/superset/sql_lab.py", line 185, in execute_sql
    db_engine_spec.execute(cursor, query.executed_sql, async_=True)
  File "/home/mbeauchemin/incubator-superset/superset/db_engine_specs.py", line 376, in execute
    cursor.execute(query)
  File "/home/mbeauchemin/incubator-superset/env/lib/python3.4/site-packages/google/cloud/bigquery/dbapi/cursor.py", line 157, in execute
    raise exceptions.DatabaseError(self._query_job.errors or e)
google.cloud.bigquery.dbapi.exceptions.DatabaseError: 400 GET https://www.googleapis.com/bigquery/v2/projects/fsql-production/queries/6ed337a4-e23c-4634-ba94-15254648bdfb?maxResults=0&location=US: Cannot query over table 'fsql-production.records.routes_stream' without a filter over column(s) 'created_at' that can be used for partition elimination

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[sumedhsakdeo]

Looks great. CC: @tswast

[googlebot]

CLAs look good, thanks!

[mistercrunch]

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