js-on-sx: raise JS TypeError for non-callable callee, undefined()

Calling a non-callable raised an OCaml-level Eval_error "Not callable"
that JS try/catch couldn't intercept. Added a (js-function? callable)
precheck in js-apply-fn that raises a TypeError instance via
(js-new-call TypeError (list msg)) so e instanceof TypeError is
true. Same swap for the undefined() branch in js-call-plain (was
raising a bare string). built-ins/String: 71/99 → 73/99 (canonical),
74/99 → 75/99 (isolated). conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,22 +1,22 @@
 {
   "totals": {
-    "pass": 71,
-    "fail": 22,
+    "pass": 73,
+    "fail": 20,
     "skip": 1,
     "timeout": 6,
     "total": 100,
     "runnable": 99,
-    "pass_rate": 71.7
+    "pass_rate": 73.7
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 100,
-      "pass": 71,
-      "fail": 22,
+      "pass": 73,
+      "fail": 20,
       "skip": 1,
       "timeout": 6,
-      "pass_rate": 71.7,
+      "pass_rate": 73.7,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
@@ -31,11 +31,11 @@
           6
         ],
         [
-          "Unhandled: Not callable: \\\\\\",
-          2
+          "ReferenceError (undefined symbol)",
+          1
         ],
         [
-          "ReferenceError (undefined symbol)",
+          "SyntaxError (parse/unsupported syntax)",
           1
         ]
       ]
@@ -54,10 +54,6 @@
       "Timeout",
       6
     ],
-    [
-      "Unhandled: Not callable: \\\\\\",
-      2
-    ],
     [
       "ReferenceError (undefined symbol)",
       1
@@ -68,6 +64,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 375.3,
+  "elapsed_seconds": 280.3,
   "workers": 1
 }