Exception raised when a key is not found in the h5 file.
If some key is not present while reading the h5 file, no exception should be raised to the user. Simply the default value is kept. Even if it is a warning, this exception blocks the system. This may occur when the structures in the h5 files are modified. For instance, the key DIR in PYHSTEXE is not used anymore and can be removed. Old h5 files should be readable even if the system and the structures have been updated.