Have the log.error() in POS log only the error message, not the full
stack trace.
Replace the global log at error - use the InvalidOperationException
code.logAtImportance()
Fix the call to logAtImportance() to avoid unexpected error, 
Add a new test to show (even manually) the process is working as
expected.
merge tjoneslo: Make pgerror logging less invasive

Merge pull request #1535 from tjoneslo/sql-layer/make-pgerror-logging-less-invasive
https://github.com/FoundationDB/sql-layer/pull/1535