Raise a proper exception, not a string

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5247

tools/ade

@@ -547,7 +547,7 @@ class SysrootManager:
             os.makedirs(dest, exist_ok=True)
             filename = tempfile.NamedTemporaryFile(dir=dest).name
         except Exception as e:
-            raise("Couldn't create download directory: {0}".format(e))
+            raise SysrootManagerError("Couldn't create download directory: {0}".format(e))
 
         try:
             hook = None