fix source location reporting for compiled code

* module/system/repl/repl.scm (default-catch-handler): Cosmetic
  improvements on VM error backtraces.

* module/system/vm/frame.scm (print-frame): Cosmetic improvements.
  (frame-line-number): source:line operates on the handle, not the cdr.
  (print-frame-chain-as-backtrace): Cosmetic improvements.

module/system/repl/repl.scm

@@ -61,9 +61,9 @@
     ((quit . _)
      (apply throw args))
     ((vm-error ,fun ,msg ,args)
-     (display "VM error: ")
-     (apply format #t msg args)
      (vm-backtrace (the-vm))
+     (display "\nVM error: \n")
+     (apply format #t msg args)
      (newline))
     ((,key ,subr ,msg ,args . ,rest)
      (vm-backtrace (the-vm))

module/system/vm/frame.scm

@@ -73,7 +73,7 @@
 
 (define (frame-line-number frame)
   (let ((addr (frame-address frame)))
-    (cond ((assv-ref (program-sources (frame-program frame)) addr)
+    (cond ((assv addr (program-sources (frame-program frame)))
            => source:line)
           (else (format #f "@~a" addr)))))
 
@@ -85,7 +85,7 @@
             "current input"))))
 
 (define (print-frame frame)
-  (format #t "~4@a: ~a ~a\n" (frame-line-number frame) (frame-number frame)
+  (format #t "~4@a: ~a   ~a\n" (frame-line-number frame) (frame-number frame)
           (frame-call-representation frame)))
 
 
@@ -112,7 +112,7 @@
         (format #t "VM backtrace:\n")
         (fold (lambda (frame file)
                 (let ((new-file (frame-file frame file)))
-                  (if (not (eqv? new-file file))
+                  (if (not (equal? new-file file))
                       (format #t "In ~a:\n" new-file))
                   (print-frame frame)
                   new-file))