How to call another function, if one function gives an error message

Question

Suppose I call a function which is named as foo.

Sometimes that function foo will give a message such as "Example of error message". If that is the case, then something else should be executed.

This means that I need a way that to read the output of the messages buffer. In particular, I want to define a function bar that calls foo and executes something when foo gives a error:

(defun bar () 
   (foo)
   (if (string-match `"Example of error message"` (minibuffer-output after calling foo))
           then do this)
      else do this thingy). 

When I read the Emacs Documentation, I couldn't find anything that gives the functionality of reading the latest message from the minibuffer.

Any suggestion would be appreciated.

SITUATION FROM THE REAL WORLD:

It seems my post is not clear enough. I will detail it further from the actual situation here. I have the following:

(defun foo () 
   (interactive)
   (evil-goto-mark ?8)
)

Calling the function foo produces the error Marker 8' is not set in this buffer.

If I get that message after calling (evil-goto-mark ?8), I want to call (evil-set-marker ?8) instead. The answer below didn't work in my case, unfortunately.

Answer 1

The error in question is thrown with user-error, not simply echoed with message.

In your particular case testing the return value of evil-get-marker should work also:

(if (evil-get-marker ?8)
  (evil-goto-mark ?8)
 (evil-set-marker ?8))

Answer 2

No need to read *Messages*. Here's how error handling works in Elisp:

(defun bar ()
  (error "asdf"))

(defun foo ()
  (condition-case e
      (bar)
    (error
     (message "Caught an error: %S" e))))

Answer 3

If the function your are calling really echoes a message, you can use (current-message).

Return the string currently displayed in the echo area, or nil if none.

Of course, it would be better to issue an error, see other answer for more details on this.