Up to this point of the tutorial, you used the original Smalltalk-80 error signalling mechanism:
check: num [ | c | c := history at: num ifAbsent: [ ^self error: 'No such check #' ]. ^c ]
In the above code, if a matching check number is found, the method will answer the object associated to it. If no prefix is found, Smalltalk will unwind the stack and print an error message including the message you gave and stack information.
CheckingAccount new: 31 "<0x33788>" error: No such check # ...blah blah... CheckingAccount>>#error:  in Dictionary>>#at:ifAbsent: Dictionary(HashedCollection)>>#findIndex:ifAbsent: Dictionary>>#at:ifAbsent:  in CheckingAccount>>#check: CheckingAccount>>#check: UndefinedObject(Object)>>#executeStatements
Above we see the object that received the #error: message, the message
text itself, and the frames (innermost-first) running when the error was
captured by the system. In addition, the rest of the code in methods
CheckingAccount>>#check: was not executed.
So simple error reporting gives us most of the features we want:
However, there is a more powerful and complex error handling mechanism,
that is exception. They are like "exceptions" in other programming
languages, but are more powerful and do not always indicate error
conditions. Even though we use the term "signal" often with regard
to them, do not confuse them with the signals like
SIGINT provided by some operating systems; they are a different
Deciding to use exceptions instead of
#error: is a matter of
aesthetics, but you can use a simple rule: use exceptions only if you want
to provide callers with a way to recover sensibly from certain errors,
and then only for signalling those particular errors.
For example, if you are writing a word processor, you might provide the
user with a way to make regions of text read-only. Then, if the user
tries to edit the text, the objects that model the read-only text can
ReadOnlyText or other kind of exception, whereupon the
user interface code can stop the exception from unwinding and report
the error to the user.
When in doubt about whether exceptions would be useful, err on the side
of simplicity; use
#error: instead. It is much easier to convert an
#error: to an explicit exception than to do the opposite.