SOLR-17456: TransactionLog ctor integrity

[dsmiley]

The TransactionLog constructor can't handle an existing file being present; it shouldn't be there. Should throw an exception in this case, NOT log a warning, leaving the object in a partially constructed state.

https://issues.apache.org/jira/browse/SOLR-17456

BTW I experienced this on a fork of Solr and I don't know the root cause. Since applying the fix in this PR, I have not seen it since (perhaps by chance). It should never happen!

Not sure if needs CHANGES.txt