Here are the of defined return codes:
200: //no error occurred
300 //no data returned but the query executed successfully, e.g selecting from empty table
1044: //The provided credentials (username) cannot be used to connnect to the database
1045: //The provided credentials (password or username) cannot be used to connnect to the database
2002 ://cannot reach the database server
4001: //Unrecognized characters. Please refer to documentation on how to insert a record
4004: //unrecognized parameter options in the insert values
4005: //operation unsuccessful.Columns count does not equal the values count
5000://Table name was not provided
6050: //Invalid sort method in orderby function
6000: //Parameter limit should be numeric function get()
6001: //Parameter offset should be numeric in function get()
6500: //Associative array expected in update function but another param data type passed
6501: //Associative array expected in update function but sequential array passed
7000 ://Invalid condition provided in where function
7001://Invalid parameters provided in where function
Also, PDO error codes and respective messages can also be returned. For documentation on such, check the official documentation here