Give a reasonable error for old data format
When a fresh `tmt` is used with older runs stored on disk, parsing
the `guests.yaml` file might fail because of missing class info.
Give a reasonable error and suggest to clean up old runs.
Fix
@@ -1379,6 +1379,11 @@ class SerializableContainer:
from tmt.plugins import import_
# Unpack class info, to get nicer variable names
+ if "__class__" not in serialized:
+ raise GeneralError(
+ "Failed to load saved state, probably because of old data format.\n"
+ "Use 'tmt clean runs' to clean up old runs.")
+
klass_info = serialized.pop('__class__')
klass_module_name = klass_info['module']
klass_name = klass_info['name']
