Do not swallow exceptions in BaseFileFormat.__exit__()

According to the context manager documentation, returning a non-False value
prevents exceptions raised inside the context manager to bubble up and being
swallowed instead. This (wrongly) prevented KeyError from being raised up to
the callers of PlainStore.set_password() when trying to change passwords for
nonexistent users.
Do not raise KeyError when deleting users without extrainfos

KeyError was being incorrectly raised when trying to delete the extrainfo
associated with an user, when the user existed but it had no extrainfo.