feat: include LegacyPandasError in init imports (#2014)

Author: shollyman

google/cloud/bigquery/__init__.py

@@ -44,6 +44,7 @@
 from google.cloud.bigquery.enums import SqlTypeNames
 from google.cloud.bigquery.enums import StandardSqlTypeNames
 from google.cloud.bigquery.exceptions import LegacyBigQueryStorageError
+from google.cloud.bigquery.exceptions import LegacyPandasError
 from google.cloud.bigquery.exceptions import LegacyPyarrowError
 from google.cloud.bigquery.external_config import ExternalConfig
 from google.cloud.bigquery.external_config import BigtableOptions