Exit and do not print backtrace on AsmError
Remove alternate machine_code implementation
Always check for file name & line number in stderr
Update to-do list
Require input string in Malformed integer error
Remove 'S' in spelling constant list name
Include input string in error message & refactor
Implement instruction? method
Raise AsmError w/message if first word is invalid
Handle errors in an included file correctly

Report correct file name & line number when error is in included file