Throw exception when attempting to read non-existant key in local storage

[pling-scottlogic]

Description

In LocalLayoutPersistenceManager, whenever we attempt to read a key from local storage which is not present, we immediately default to returning an empty array. We want to change this behaviour so that an exception is thrown, then caught and handled outside of the persistence manager.

Work Required

• In LocalLayoutPersistenceManager, throw an appropriate exception whenever getLocalEntity returns undefined
• Catch these exceptions outside of LocalLayoutPersistenceManager and handle appropriately (e.g. defaulting to an empty array)

[pling-scottlogic]

Happy with the way LocalLayoutManager is currently handling this. Added a note to #27 to handle remote errors appropriately.