Loads JSON from disk.
In the case where the file doesn't exist or is otherwise invalid, then JSONValue is initialised to null (by way of JSONStorage.reset).
Filename of file to read from.
Whatever readText and/or parseJSON throws.
lu.common.FileTypeMismatchException if the filename exists but is not a file.
See Implementation
Loads JSON from disk.
In the case where the file doesn't exist or is otherwise invalid, then JSONValue is initialised to null (by way of JSONStorage.reset).