Fix a crash in the monitor.

This patch set fixes a crash in the monitor loop in case of receiving an STH
with just an increased timestamp (i.e. tree size does not change).

It also logs non-critical network problems while fetching STHs.
Make the "result" local variable const.
Remove the GetResultString static method.

It is used only in one general place, and is mostly duplication of what's already at its call sites.
Log "result" in the case where it is unknown.
Make the "sanity" local variable const.