Improper error messages are caused by calling exit(n) with an invalid or
missing return value n.  Assembler programmers using startup code should
jump to the startup code's _exit with a valid return value on the stack.
Programs without startup code should return with a valid value in D0.
Valid return values such as RETURN_OK, RETURN_WARN, RETURN_FAIL are
defined in <dos/dos.h> and <dos/dos.i>.  values outside of these ranges
(-1 for instance) can cause invalid CLI error messages such as "not an
object module".  Useful hint--if your program is called from a script,
your valid return value can be conditionally branched on in the script
(i.e., call program, then perform actions based on IF WARN or IF NOT
WARN).  RETURN_FAIL will cause the script to stop if a normal FAILAT value
is being used in script.