make mxerr more robust

src/backend/mxnet/graph.jl

@@ -105,6 +105,7 @@ macro mxerr(stk, ex)
     catch e
       (isa(e, mx.MXError) && (node = errnode(e)) != nothing) || rethrow()
       stk = $(esc(stk))
+      haskey(stk, node) || rethrow()
       throw(Exception(e, totrace(stk[node])))
     end)
 end

test/backend.jl

@@ -25,6 +25,7 @@ end
   e = try mxnet(model, (10, 1))
   catch e e end
 
+  @test isa(e, DataFlow.Interpreter.Exception)
   @test e.trace[1].func == Symbol("Flux.Affine")
   @test e.trace[2].func == :TLP
 end