Erreur Additional uncaught exception thrown while handling exception.

Bonsoir,
je voudrais installer drupal 8 beta 11 sous debian wheezy avec comme SGBQ posgress 9.4. mais à l'exécution du script apres avoir entré le nom de la base de donnée et le mot de passe utilisateur de cette base de donnée, j'ai l'erreur suivante.
Je vous prie de me en en aide.

Additional uncaught exception thrown while handling exception.

Original

Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[42P07]: Duplicate table: 7 ERROR: relation "d811cache_discovery" already exists: CREATE TABLE {cache_discovery} ( "cid" varchar(255) NOT NULL default '', "data" bytea NULL, "expire" int NOT NULL default 0, "created" numeric(14, 3) NOT NULL default 0, "serialized" smallint NOT NULL default 0, "tags" text NULL, "checksum" varchar(255) NOT NULL, CONSTRAINT D811cache_discovery____pkey PRIMARY KEY ("cid") ); Array ( ) in Drupal\Core\Plugin\DefaultPluginManager->cacheSet() (line 236 of /var/www/drupal/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php).

Drupal\Core\Plugin\DefaultPluginManager->cacheSet('entity_type', Array, -1, Array)
Drupal\Core\Plugin\DefaultPluginManager->setCachedDefinitions(Array)
Drupal\Core\Plugin\DefaultPluginManager->getDefinitions()
Drupal\Core\Config\ConfigManager->getEntityTypeIdByName('core.extension')
Drupal\Core\Config\ConfigInstaller->createConfiguration('', Array)
Drupal\Core\Config\ConfigInstaller->installDefaultConfig('core', 'core')
drupal_install_system(Array)
install_base_system(Array)
install_run_task(Array, Array)
install_run_tasks(Array)
install_drupal(Object)
Additional

Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[42P01]: Undefined table: 7 ERROR: relation "d811key_value" does not exist LINE 3: D811key_value key_value ^: SELECT 1 AS expression FROM {key_value} key_value WHERE ( (name = :db_condition_placeholder_0) AND (collection = :db_condition_placeholder_1) ); Array ( [:db_condition_placeholder_0] => system.theme.files [:db_condition_placeholder_1] => state ) in Drupal\Core\State\State->set() (line 89 of /var/www/drupal/core/lib/Drupal/Core/State/State.php).

Drupal\Core\State\State->set('system.theme.files', Array)
Drupal\Core\Extension\ThemeHandler->rebuildThemeData()
Drupal\Core\Extension\ThemeHandler->refreshInfo()
Drupal\Core\Extension\ThemeHandler->listInfo()
_drupal_maintenance_theme()
drupal_maintenance_theme()
_drupal_log_error(Array, 1)
_drupal_exception_handler(Object)