Fix IDT error # bug, add debug helper, continue debugging spawn_thread crash

The crash occurs after the newly-created stack is accessed, suggesting
that there is some bug in the paging code.
diff --git a/src/kernel/idt.s b/src/kernel/idt.s
index 1b70054..5ed637e 100644
--- a/src/kernel/idt.s
+++ b/src/kernel/idt.s
@@ -40,11 +40,11 @@
 ISRERR 14
 ISRNOERR 15
 ISRNOERR 16
-ISRNOERR 17
+ISRERR 17
 ISRNOERR 18
 ISRNOERR 19
 ISRNOERR 20
-ISRNOERR 21
+ISRERR 21
 ISRNOERR 22
 ISRNOERR 23
 ISRNOERR 24