ACCUMULO-2870 Fail hard and fast if we don't calculate a midpoint.

By failing early, we catch the change in functionality from FileUtil.findMidPoint
which always returns a mid-point. Also prevents an NPE from unexpectedly being thrown.
Merge branch '1.5.2-SNAPSHOT' into 1.6.1-SNAPSHOT

Conflicts:
	server/tserver/src/main/java/org/apache/accumulo/tserver/Tablet.java
Merge branch '1.6.1-SNAPSHOT'