I attach a patch for ielm.el that makes it possible to enter the debugger
when executing expressions in ielm, and that makes ielm respect the
debug-on-error and debug-on-exit flags.

I guess that possibly the reason that was not originally done is that it is
not obvious how to restore ielm to a usable state after the debugger has
been entered. My patch makes use of the fact that unwind-protect unwind
forms are still executed when user resumes execution from the debugger
after an error.

Without the patch, ielm wraps the evaluation of the expression given by the
user in a condition case, and in case of an error, or exit, displays an
appropriate message in its buffer, right under the evaluated expression,
regardless of debug-on-error and debug-on-exit.

With the patch, the message that ielm displays in its buffer will be a
generic error message  regardless if there was an error in the evaluated
form, or a quit. However:

- if debug-on-error is t, emacs will enter the debugger and show a stack
trace, just like with almost any other evaluation method. When the user
continues the execution from the debugger, ielm will correctly resume
execution

- if debug-on-error is nil, emacs will display the specific error in the
minibuffer anyway

Similar things are true for debug-on-quit.

I think this is an improvement over the current state of affairs. Of course
ideally I would like to preserve the "nice" message in ielm buffer and make
the improvements I made, but emacs does not seem to make it possible to do
some handling of an error and then to re-raise it while preserving the
original stack trace.

Cheers,
Jarosław Rzeszótko