bigquery#insert: return & test API errors. fixes #336

RE: #336 

This tests that insert errors are returned from the API during a BigQuery table insert action.
Yeah, err would be an API error, while insertErrors would contain any errors that occurred during the insert. For example, if a connection to the API fails, `err` is populated and nothing worked. If the connection is fine, and the API request completes successfully, there could still have been issues with inserting some of the data.