datalog: tokenizer raises on unterminated string + quoted atom (251/251)

Bug: read-quoted ran to EOF silently when the closing quote was
missing. The token's value was whatever ran-to-end string had been
accumulated; the parser later saw an unexpected EOF, but the error
message blamed the wrong location ("expected `)` got eof") and
hid the real problem.

Fix: read-quoted now raises with a message that distinguishes
strings from quoted atoms, including the position where the
opening quote was lost. The escape-sequence handling and proper
closing are unaffected.

2 new tokenize tests.

lib/datalog/tokenizer.sx

@@ -123,7 +123,11 @@
               (fn
                 ()
                 (cond
-                  ((>= pos src-len) nil)
+                  ((>= pos src-len)
+                    (error
+                      (str "Tokenizer: unterminated "
+                           (if (= quote-char "'") "quoted atom" "string")
+                           " (started near position " pos ")")))
                   ((= (cur) "\\")
                     (do
                       (advance! 1)