*scratch*.el

(cl-defun my-custom-install (package &key then error)
  "Install PACKAGE if not installed and configure it.

key :then specifies a function to run when package installs successfully or is already installed.
key :error specifies a function to run when package installation fails
    this function recieves one argument, an error object."
  (declare (indent defun))
  (condition-case err
      (progn (unless (package-installed-p package)
               (package-install package))
             (when then (funcall then)))
    (error (when error (funcall error err)))))



(my-custom-install 'helm
  :then (lambda ()
          (message "Configuratin'!"))
  :error (lambda(err) (message "OH NO! %s" (error-message-string err)))) ;; => "Configuratin'!"


(my-custom-install 'not-a-package
  :then (lambda ()
          (message "Configuratin'!"))
  :error (lambda(err) (message "OH NO! %s" (error-message-string err)))) ;; =>  "OH NO! Package `not-a-package' is not available for installation"