bigquery: unpredictable or poorly documented errors for checking if a table exists

**Is your feature request related to a problem? Please describe.**

When using to bigquery package it is important to check if a table exists or not before starting to insert rows into it. To do this, the doc string for dataset.Table() recommends:

```
Table creates a handle to a BigQuery table in the dataset. To determine if a table exists, call Table.Metadata. If the table does not already exist, use Table.Create to create it.
```

If you call Metadata() on a table pointer that is returned from dataset.Table() you get a Metadata pointer and an error. If that table doesn't exist the metadata pointer is nil and the error reads: `googleapi: Error 404: Not found: Table , notFound`

However, how can I verify that the error is because of the table not being found, and not something like a dropped network connection?

**Describe the solution you'd like**

There should be an error defined by the bigquery client library such as `bigquery.TableNotFound`

**Describe alternatives you've considered**

The only option I see here is to string search for "404" or "Not found"

**Additional context**