doAssertRaises improvements; nimscript supports except Exception as e (#15765)

* doAssertRaises now correctly handles foreign exceptions; now shows which exception is raised on mismatch
* nimscript now handles `Exception as e`
* remove catch-all doAssertRaises overload from this PR

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

lib/system.nim

@@ -2360,7 +2360,8 @@ when notJSnotNims and hostOS != "standalone":
     ##
     ## **Warning**: Only use this if you know what you are doing.
     currException = exc
-
+elif defined(nimscript):
+  proc getCurrentException*(): ref Exception {.compilerRtl.} = discard
 
 when notJSnotNims:
   {.push stackTrace: off, profiler: off.}