Log text of failed queries at log level ERROR

Previously query texts were always logged at log level DEBUG, but
that doesn't help much in a normal production environment when
trying to identify the cause of issues.

Also make various other minor improvements to query logging and
handling of database errors.

Implements GitHub #498.

doc/appendix-release-notes.sgml

@@ -17,7 +17,7 @@
 
   <sect1 id="release-4.1.1">
     <title>Release 4.1.1</title>
-    <para><emphasis>??? ??? ??, 2018</emphasis></para>
+    <para><emphasis>Wed September 5, 2018</emphasis></para>
     <para>
     </para>
 
@@ -33,6 +33,14 @@
             </para>
           </listitem>
 
+          <listitem>
+            <para>
+              Text of any failed queries will now be logged as <literal>ERROR</literal> to assist
+              logfile analysis at log levels higher than <literal>DEBUG</literal>.
+              (GitHub #498).
+            </para>
+          </listitem>
+
         </itemizedlist>
       </para>
    </sect2>