add more descriptive message to the exception, will be displayed to the user

apps/encryption/lib/session.php

@@ -70,7 +70,7 @@ class Session {
 	public function getPrivateKey() {
 		$key = $this->session->get('privateKey');
 		if (is_null($key)) {
-			throw new Exceptions\PrivateKeyMissingException('no private key stored in session', 0);
+			throw new Exceptions\PrivateKeyMissingException('please try to log-out and log-in again', 0);
 		}
 		return $key;
 	}