Improve reporting of missing blocks / entries when syncing

[mberry]

This error msg could be cleaned up or perhaps handled a bit better:

[WhoSoup]

Yeah, that's been a discussion for a while now. To summarize, this is caused by the connected factomd node reporting that they have reached a certain height, but not having all of the data downloaded yet (particularly entries).

One potential fix is to use entryblockheight instead of directoryblockheight when syncing.