If an error occurs in the .init or .check procedure, the erroneous command will be returned to the user, and the procedure execution is interrupted by a pause. You can then correct the error, execute the command, and type C or CONTINUE to resume the procedure execution. Or you can type QUIT (as in any SIC procedure indeed) to abort the execution, until the RUN or SUBMIT command returns an error.
You may also want to abort a RUN or SUBMIT command while you are in the editor: typing QUIT instead of EXIT to end the editing will do it.