CLJS-1907: Improve error message from cljs.reader/read-string

When reading keywords with numbers first. Before this patch, re-matches*
would return nil, and then the aget for token would throw an error that
null is not an object.

Author: danielcompton

src/main/cljs/cljs/reader.cljs

@@ -365,6 +365,7 @@ nil if the end of stream has been reached")
   [reader initch]
   (let [token (read-token reader (read-char reader))
         a (re-matches* symbol-pattern token)
+        _ (when (nil? a) (reader-error reader "Invalid keyword " (str ":" token)))
         token (aget a 0)
         ns (aget a 1)
         name (aget a 2)]

src/test/cljs/cljs/reader_test.cljs

@@ -223,3 +223,7 @@
     (is (= "Duplicate key: :a" b))
     (is (= "Duplicate key: :a" c))
     (is (= "Duplicate key: :a" d))))
+
+(deftest test-error-messages
+  (testing "Leading numbers in keywords"
+    (is (thrown-with-msg? js/Error #"Invalid keyword :0s" (reader/read-string ":0s")))))