Make print-backtrace-and-exception/knots more reliable

Catch failures in the Guile code for printing backtraces, and print
the partial backtrace, plus the backtrace of the exception that
happened.

knots.scm

@@ -67,19 +67,10 @@
 (define* (print-backtrace-and-exception/knots
           exn
           #:key (port (current-error-port)))
-  (let* ((stack
+  (define (get-string port stack)
-          (match (fluid-ref %stacks)
+    (define stack-len
-            ((stack-tag . prompt-tag)
-             (make-stack #t
-                         0 prompt-tag
-                         0 (and prompt-tag 1)))
-            (_
-             (make-stack #t))))
-         (stack-len
       (stack-length stack))
-         (error-string
+
-          (call-with-output-string
-            (lambda (port)
     (let ((knots-stacks
            (map knots-exception-stack
                 (filter knots-exception?
@@ -126,5 +117,35 @@
                             stack-len
                             3))))
        '%exception
-                 (list exn)))))))
+       (list exn))))
+
+  (let* ((stack
+          (match (fluid-ref %stacks)
+            ((stack-tag . prompt-tag)
+             (make-stack #t
+                         0 prompt-tag
+                         0 (and prompt-tag 1)))
+            (_
+             (make-stack #t))))
+         (string-port
+          (open-output-string))
+         (error-string
+          (with-exception-handler
+              (lambda (exn)
+                (display (get-output-string string-port)
+                         port)
+                (close-output-port string-port)
+                (display "\n\n" port)
+
+                (backtrace port)
+                (simple-format
+                 port
+                 "\nexception in print-backtrace-and-exception/knots: ~A\n"
+                 exn)
+                (raise-exception exn))
+            (lambda ()
+              (get-string string-port stack)
+              (let ((str (get-output-string string-port)))
+                (close-output-port string-port)
+                str)))))
     (display error-string port)))