Code review for escape-strings patch. Sync psql and plpgsql lexers

Code review for escape-strings patch.  Sync psql and plpgsql lexers
with main, avoid using a SQL-defined SQLSTATE for what is most definitely
not a SQL-compatible error condition, fix documentation omissions,
adhere to message style guidelines, don't use two GUC_REPORT variables
when one is sufficient.  Nothing done about pg_dump issues.