backtrace: keep old backtraces on reraise

The doccoment makes it look like the backtrace is kept, but this is not
the case. Use is_important to make sure this happens.

Signed-off-by: Pau Ruiz Safont <pau.safont@vates.tech>

Author: psafont

lib/backtrace.ml

@@ -246,6 +246,7 @@ let get exn =
   |> Option.fold ~some:(fun tbl -> get tbl exn) ~none:empty
 
 let reraise old newexn =
+  is_important old;
   add newexn (remove old);
   raise newexn
 

test/reraise.t

@@ -6,8 +6,10 @@
   $ ./raiser.exe -reraise
   Backtrace lab failed with exception Failure("bar")
   Raised Failure("bar")
-  1/2 raiser.exe Raised at file lib/backtrace.ml, line 250
-  2/2 raiser.exe Called from file test/log.ml, line 55
+  1/4 raiser.exe Raised at file test/raiser.ml, line 1
+  2/4 raiser.exe Called from file test/raiser.ml, line 4
+  3/4 raiser.exe Called from file lib/backtrace.ml, line 251
+  4/4 raiser.exe Called from file test/log.ml, line 55
 
 
   $ ./raiser.exe -v1-with-backtrace