SpannerQueryResult: Change 'error' to 'err'

All consumers of SpannerQueryResult expect `err` instead of `error`.
Without this fix, failures are silent. With this, they're displayed
within the notebook.

Thanks to @cqian23 for pointing this out

Author: Sailesh Mukil

spanner_graphs/cloud_database.py

@@ -117,7 +117,13 @@ def execute_query(
                 results = snapshot.execute_sql(query, params=params, param_types=param_types)
                 rows = list(results)
             except Exception as e:
-                return {}, [], [], self.schema_json, e
+                return SpannerQueryResult(
+                    data={},
+                    fields=[],
+                    rows=[],
+                    schema_json=self.schema_json,
+                    err=e
+                )
 
             fields: List[SpannerFieldInfo] = get_as_field_info_list(results.fields)
             data = {field.name: [] for field in fields}
@@ -128,7 +134,7 @@ def execute_query(
                     fields=fields,
                     rows=rows,
                     schema_json=self.schema_json,
-                    error=None
+                    err=None
                 )
 
             for row_data in rows:
@@ -143,5 +149,5 @@ def execute_query(
                 fields=fields,
                 rows=rows,
                 schema_json=self.schema_json,
-                error=None
+                err=None
             )

spanner_graphs/database.py

@@ -37,7 +37,7 @@ class SpannerQueryResult(NamedTuple):
     # An optional field to return the schema as JSON
     schema_json: Any | None
     # The error message if any
-    error: Exception | None
+    err: Exception | None
 
 class SpannerDatabase(ABC):
     """The spanner class holding the database connection"""
@@ -129,7 +129,7 @@ def execute_query(
                     fields=fields,
                     rows=rows,
                     schema_json=self.schema_json,
-                    error=None
+                    err=None
                 )
 
         for i, row in enumerate(results):
@@ -143,5 +143,5 @@ def execute_query(
                 fields=fields,
                 rows=rows,
                 schema_json=self.schema_json,
-                error=None
+                err=None
             )

spanner_graphs/graph_server.py

@@ -25,7 +25,7 @@
 
 from spanner_graphs.conversion import get_nodes_edges
 from spanner_graphs.exec_env import get_database_instance
-
+from spanner_graphs.database import SpannerQueryResult
 
 # Supported types for a property
 PROPERTY_TYPE_SET = {