Final review pass.

Using `assertRaises` instead of a `self.fail()` branch that
will never get triggered.

Also moving around imports in `google.cloud.spanner_v1.database`
to be alphabetical / separated by 1st-party and 3rd-party.

spanner/google/cloud/spanner_v1/database.py

@@ -17,20 +17,20 @@
 import re
 import threading
 
-import google.auth.credentials
 from google.api_core import exceptions
+import google.auth.credentials
+from google.cloud.exceptions import Conflict
+from google.cloud.exceptions import NotFound
 from google.gax.errors import GaxError
 from google.gax.grpc import exc_to_code
-from google.cloud.spanner_v1.gapic.spanner_client import SpannerClient
 from grpc import StatusCode
 import six
 
 # pylint: disable=ungrouped-imports
-from google.cloud.exceptions import Conflict
-from google.cloud.exceptions import NotFound
 from google.cloud.spanner_v1 import __version__
 from google.cloud.spanner_v1._helpers import _options_with_prefix
 from google.cloud.spanner_v1.batch import Batch
+from google.cloud.spanner_v1.gapic.spanner_client import SpannerClient
 from google.cloud.spanner_v1.pool import BurstyPool
 from google.cloud.spanner_v1.pool import SessionCheckout
 from google.cloud.spanner_v1.session import Session

spanner/tests/system/test_system.py

@@ -304,13 +304,11 @@ def test_table_not_found(self):
                 index,
             ],
         )
-        try:
+        with self.assertRaises(NotFound) as exc_info:
             temp_db.create()
-        except NotFound as exc:
-            self.assertEqual(exc.args[0],
-                             'Table not found: {0}'.format(incorrect_table))
-        except:
-            self.fail()
+
+        expected = 'Table not found: {0}'.format(incorrect_table)
+        self.assertEqual(exc_info.exception.args, (expected,))
 
     def test_update_database_ddl(self):
         pool = BurstyPool()