stacktrace fixes

src/backend/mxnet/graph.jl

@@ -9,6 +9,7 @@ end
 using Base: @get!
 using DataFlow: Constant, constant
 using DataFlow.Interpreter
+using DataFlow.Interpreter: Exception, totrace
 using Flux: imap
 
 # TODO: implement Julia's type promotion rules
@@ -97,6 +98,6 @@ macro mxerr(stk, ex)
     catch e
       (isa(e, mx.MXError) && (node = errnode(e)) != nothing) || rethrow()
       stk = $(esc(stk))
-      throw(DataFlow.Exception(e, DataFlow.totrace(stk[node])))
+      throw(Exception(e, totrace(stk[node])))
     end)
 end