Fix AttributeError when pool is None in Bolt3 error handling

Add null check for self.pool before calling on_neo4j_error() in
async Bolt3 implementation to prevent AttributeError when the
connection pool is not available during Neo4jError handling.

Author: kirill-stepanishin

src/neo4j/_async/io/_bolt3.py

@@ -588,7 +588,8 @@ async def _process_message(self, tag, fields):
                     )
                 raise
             except Neo4jError as e:
-                await self.pool.on_neo4j_error(e, self)
+                if self.pool:
+                    await self.pool.on_neo4j_error(e, self)
                 raise
         else:
             sig_int = ord(summary_signature)