installer: Make dump archive creation optional and selective.

installer: Make dump archive creation optional and selective.

* gnu/installer.scm (installer-program): Let the installer customize
the dump archive.
* gnu/installer/dump.scm (prepare-dump, make-dump): Split make-dump in
prepare-dump, which copies the files necessary for the dump, and
make-dump which creates the archive.
* gnu/installer/record.scm (installer): Add report-page field.  Change
documented return value of exit-error.
* gnu/installer/newt.scm (exit-error): Change arguments to be a string
containing the error.  Let the user choose between exiting and
initiating a dump.
(report-page): Add new variable.
* gnu/installer/newt/page.scm (run-dump-page): New variable.
* gnu/installer/newt/dump.scm: Delete it.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>