Handle channel exceptions

Channel errors can be trapped with the WHENEVER ERROR exception handler:

WHENEVER ERROR CONTINUE
CALL ch.write([num,label])
IF STATUS THEN
  ERROR "An error occurred while reading from Channel"
  CALL ch.close()
  RETURN -1
END IF
WHENEVER ERROR STOP

Or with a TRY/CATCH block:

TRY
  CALL ch.write([num,label])
CATCH
  ERROR "An error occurred while reading from Channel"
  CALL ch.close()
  RETURN -1
END TRY