Prior to this change the exception on line 138 could not
be thrown, as far as I can tell. The reason for this
is that`HeaderPrefix` contained a trailing space, and we
had verified that `line` (trimmed) started with
`HeaderPrefix`. Hence `line` must contain something more
than `HeaderPrefix` otherwise the tailing space would
have been removed, so the length of `line` could not be
less than or equal to the length of `HeaderPrefix`.
Added and changed tests to ensure that both exceptions
regarding `FormatVersion` are thrown.
Fixes#5978
