Update does not fail when snapshot is not valid

[renatav]

When timestamp is invalid, update fails. If snapshot is updated in the same invalid way, it does not.

The problem here is that TUF's updater only downloads new metadata files if it is determined based on other metadata files that they should be updated. However, metadata files which are not downloaded by the TUF updater might have been changed in a particular commit and might be invalid. So, if we change snapshot without updating timestamp, TUF updater will not download it. But that snapshot metadata file could be invalid. We cannot simply check if the metadata files are the same since we have cases where we unnecessarily increased a version and signed, even though nothing was updated.