fixes #5986 js backend failed to compile try ... except new syntax (#6116)

lib/system/jssys.nim

@@ -53,7 +53,7 @@ proc isNimException(): bool {.asmNoStackFrame.} =
   else:
     asm "return `lastJSError`.m_type;"
 
-proc getCurrentException*(): ref Exception =
+proc getCurrentException*(): ref Exception {.compilerRtl, benign.} =
   if isNimException(): result = cast[ref Exception](lastJSError)
 
 proc getCurrentExceptionMsg*(): string =

tests/js/ttryexceptnewsyntax.nim

@@ -0,0 +1,13 @@
+discard """
+  output: '''hello'''
+"""
+
+type
+  MyException = ref Exception
+
+#bug #5986
+
+try:
+  raise MyException(msg: "hello")
+except MyException as e:
+  echo e.msg