replace all assert statements with Preconditions.checkState

I was bitten by an ugly bug in socialsensor/graphdb-benchmark because Titan/JG was failing silently when checking the validity of a manually-created vertex id. Actually throwing the exception will help us catch the poison pills and increase code quality.

Search the Java source for assert statements and review the surrounding validity checks before deciding which uses correspond to Preconditions.checkState. Completion means the applicable assertions are replaced with explicit checks and the existing test suite is run to identify regressions; the issue names no specific files or tests.

Written by the indexing model from the issue text.