Postgres: Config diagnostics / logging update (#105666)

* config diagnostics update

* update tests

* fix typo

pkg/tsdb/grafana-postgresql-datasource/sqleng/errors.go

@@ -3,5 +3,6 @@ package sqleng
 import "errors"
 
 var (
-	ErrInvalidPortSpecified error = errors.New("invalid port in host specifier")
+	ErrParsingPostgresURL error = errors.New("error parsing postgres url")
+	ErrCertFileNotExist   error = errors.New("certificate file doesn't exist")
 )