installer: Skip the backtrace page on user abort.

When the user aborts the installation because a core dump is discovered or the
installation command failed, displaying the abort backtrace doesn't make much
sense. Hide it when the abort condition is &user-abort-error and skip directly
to the dump page.

* gnu/installer/steps.scm (&user-abort-error): New variable.
(user-abort-error?): New procedure.
* gnu/installer/newt/final.scm (run-install-failed-page): Raise a
user-abort-error.
* gnu/installer/newt/welcome.scm (run-welcome-page): Ditto.
* gnu/installer.scm (installer-program): Hide the backtrace page and directly
propose to dump the report when the a &user-abort-error is raised.

gnu/installer/newt/final.scm

@@ -92,9 +92,8 @@ a specific step, or restart the installer."))
         ;; Keep going, the installer will be restarted later on.
         #t)
        (3 (raise
-           (condition
-            (&message
-             (message "User abort.")))))))
+            (condition
+             (&user-abort-error))))))
     (_
      (send-to-clients '(installation-failure))
      #t)))

gnu/installer/newt/welcome.scm

@@ -145,8 +145,7 @@ first?"))
         (1 #t)
         (2 (raise
             (condition
-             (&message
-              (message "User abort.")))))))
+             (&user-abort-error))))))
     (run-menu-page
      (G_ "GNU Guix install")
      (G_ "Welcome to GNU Guix system installer!

gnu/installer/steps.scm

@@ -28,7 +28,10 @@
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
   #:use-module (rnrs io ports)
-  #:export (<installer-step>
+  #:export (&user-abort-error
+            user-abort-error?
+
+            <installer-step>
             installer-step
             make-installer-step
             installer-step?
@@ -50,6 +53,9 @@
 
             %current-result))
 
+(define-condition-type &user-abort-error &error
+  user-abort-error?)
+
 ;; Hash table storing the step results. Use it only for logging and debug
 ;; purposes.
 (define %current-result (make-hash-table))