The data driver verbs that open and close connections, and send SQL statements, all return a number. Typically, if the number is positive, it is a handle. If the number is 0, it means the operation completed successfully (but the function returns no handle). If the number is _1, it means there was some error. You can get more information about the errror using dderr. For example, try closing a non-existent statement handle:
ddend 42 _1 dderr'' ISI04 Bad statement handle