Differentiate SCRIPT KILL error replies.

When calling SCRIPT KILL currently you can get two errors:

* No script in timeout (busy) state.
* The script already performed a write.

It is useful to be able to distinguish the two errors, but right now both
start with "ERR" prefix, so string matching (that is fragile) must be used.

This commit introduces two different prefixes.

-NOTBUSY and -UNKILLABLE respectively to reply with an error when no
script is busy at the moment, and when the script already executed a
write operation and can not be killed.

tests/unit/scripting.tcl

@@ -301,7 +301,7 @@ start_server {tags {"scripting"}} {
         catch {r ping} e
         assert_match {BUSY*} $e
         catch {r script kill} e
-        assert_match {ERR*} $e
+        assert_match {UNKILLABLE*} $e
         catch {r ping} e
         assert_match {BUSY*} $e
     }