Fix #114: Better error in ->clj when no kernel

Issue: Thomas reported getting to express with no jlink-instance. This should not be possible, as wl/eval that ->clj calls checks for its presence but maybe I am missing something.

Fix: Throw a helpful error if this ever happens.

Also make terminate-kernel! more robust (in case when start failed)

src/wolframite/base/express.clj

@@ -8,7 +8,10 @@
   (assert (string? s) (str "Expected '" (pr-str s) "' to be a string but is " (type s)))
   ;; TODO: debug log: "express string>"
   (let [held-s (str "HoldComplete[" s "]")
-        link (proto/kernel-link jlink-instance)
+        link (or (proto/kernel-link jlink-instance)
+                 (throw (IllegalStateException.
+                          (str "Something is wrong - there is no JLink instance, which shouldn't"
+                               " be possible. Try to stop and start Wolframite again."))))
         output (io!
                 (locking link
                   (doto link

src/wolframite/impl/jlink_proto_impl.clj

@@ -90,9 +90,10 @@
           res))))
   (terminate-kernel! [_this]
     ;; BEWARE: it is not absolutely guaranteed that the kernel will die immediately
-    (doto ^KernelLink @kernel-link-atom
-      (.terminateKernel)
-      (.close))
+    (when-let [link ^KernelLink @kernel-link-atom]
+      (doto link
+        (.terminateKernel)
+        (.close)))
     (reset! kernel-link-atom nil))
   (expr [_this primitive-or-exprs]
     (make-expr primitive-or-exprs))