Don't crash when updating a branch

This was caused by a race condition between the instance shutting down
and managetests deleting its files.

managetests/branches.py

@@ -286,7 +286,10 @@ class Branch:
         if not self.present():
             return
 
-        shutil.rmtree(str(self.manager.root / "branches" / self.name))
+        shutil.rmtree(
+            str(self.manager.root / "branches" / self.name),
+            ignore_errors=True
+        )
 
         try:
             del self.manager.details["branches"][self.name]