Don't stop parsing an ODF unless the container state becomes invalid

An error is not necessarily fatal unless the handler sets the state to
invalid as a result of the missing data. In addition, when the state
is set to invalid, log the component that failed to load and (if
defined) the error that has occurred.
Handle documents with no office:settings element

Settings.xml and office:settings are both optional according to the
ODF specifications. WebODF must be able to handle loading and saving
files with or without this setting.
Ensure all ODF elements are recreated when setting a new root element