Prevent await inside janet_call - address #1531

This was partially implemented before, but not in the case where the
await or other signal itself was created by a C function. We have to
separate code paths for generating signals - one via normal returns in
janet_vm_continue, and the other via longjump. This adds handling for
the longjump case, as well as improved messaging.

test/suite-ev.janet

@@ -465,4 +465,13 @@
 # Close chat server
 (:close chat-server)
 
+# Issue #1531
+(def c (ev/chan 0))
+(ev/spawn (while (def x (ev/take c))))
+(defn print-to-chan [x] (ev/give c x))
+(assert-error "coerce await inside janet_call to error"
+              (with-dyns [*out* print-to-chan]
+                (pp :foo)))
+(ev/chan-close c)
+
 (end-suite)