Wrap error-handler around calls to LAUNCH-NEW-INSTANCE and RESUME-URL.

This will catch most errors, but will fail on some, e.g. procedure
called with wrong number of arguments. As far as I see, the problem is
the underlying error reporting system of Scsh/Scheme48. Maybe I'm
wrong.

scheme/httpd/surflets/surflet-handler.scm

@@ -78,9 +78,19 @@
 	  (if (or (string=? request-method "GET")
 		  (string=? request-method "POST"))
 	      (let ((response 
-		     (if (resume-url? path-string)
-			 (resume-url path-string servlet-path req)
-			 (launch-new-instance path-string servlet-path req))))
+		     (call-with-current-continuation
+		      (lambda (exit)
+			(with-handler
+			 (lambda (condition more)
+			   (exit 
+			    (make-http-error-response 
+			     http-status/bad-gateway req
+			     (format #f "Internal error while executing servlet ~s." path)
+			     condition)))
+			 (lambda ()
+			   (if (resume-url? path-string)
+			       (resume-url path-string servlet-path req)
+			       (launch-new-instance path-string servlet-path req))))))))
 		(if (redirect-body? (response-body response))
 		    ;; Pefix it with servlet-prefix.
 		    (make-redirect-response