Change the behavior of the sqlite3_wal_hook() callback. It should now return

SQLITE_OK or an error code and the error code is propagated back up the 
stack.  If a checkpoint is desired, the callback should invoke
sqlite3_wal_callback() itself.

FossilOrigin-Name: 1b14195e05fe5551992a39246ec3bcf6a33bbfac

test/walhook.test

@@ -49,7 +49,7 @@ do_test walhook-1.2 {
   set ::wal_hook
 } {main 5}
 do_test walhook-1.3 {
-  proc wal_hook {args} { return 1 }
+  proc wal_hook {args} { db eval {PRAGMA wal_checkpoint}; return 0 }
   execsql { INSERT INTO t1 VALUES(2, 'two') }
   file size test.db
 } [expr 3*1024]