scalar-delete: do not 'die()' in 'delete_enlistment()'

scalar-delete: do not 'die()' in 'delete_enlistment()'

Rather than exiting with 'die()' when 'delete_enlistment()' encounters an
error, return an error code with the appropriate message. There's no need
for an abrupt exit with 'die()' in 'delete_enlistment()' because its only
caller ('cmd_delete()') properly cleans up allocated resources and returns
the 'delete_enlistment()' return value as its own exit code.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>