Add test. Fix a corresponding EROR mismatch in boot.ss

Author: dolio

scheme-libs/racket/unison/boot.ss

@@ -607,5 +607,5 @@
 (define (exn:bug->exception b)
   (raise-unison-exception
     ref-runtimefailure:typelink
-    (exn:bug-msg b)
+    (string->chunked-string (exn:bug-msg b))
     (exn:bug-val b)))

unison-src/builtin-tests/jit-tests.output.md

@@ -49,3 +49,18 @@ foo = do
   ()
 
 ```
+This can also only be tested by separately running this test, because
+it is exercising the protocol that ucm uses to talk to the jit during
+an exception.
+
+```ucm
+runtime-tests/selected> run.native testBug
+
+  💔💥
+  
+  I've encountered a call to builtin.bug with the following
+  value:
+  
+    "testing"
+
+```

unison-src/builtin-tests/jit-tests.tpl.md

@@ -34,3 +34,11 @@ foo = do
 .> run.native foo
 .> run.native foo
 ```
+
+This can also only be tested by separately running this test, because
+it is exercising the protocol that ucm uses to talk to the jit during
+an exception.
+
+```ucm:error
+runtime-tests/selected> run.native testBug
+```