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(elisp)Processing of Errors

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How Emacs Processes Errors
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   When an error is signaled, `signal' searches for an active "handler"
for the error.  A handler is a sequence of Lisp expressions designated
to be executed if an error happens in part of the Lisp program.  If the
error has an applicable handler, the handler is executed, and control
resumes following the handler.  The handler executes in the environment
of the `condition-case' that established it; all functions called
within that `condition-case' have already been exited, and the handler
cannot return to them.

   If there is no applicable handler for the error, the current command
is terminated and control returns to the editor command loop, because
the command loop has an implicit handler for all kinds of errors.  The
command loop's handler uses the error symbol and associated data to
print an error message.

   An error that has no explicit handler may call the Lisp debugger.
The debugger is enabled if the variable `debug-on-error' (Note: Error
Debugging) is non-`nil'.  Unlike error handlers, the debugger runs in
the environment of the error, so that you can examine values of
variables precisely as they were at the time of the error.