more verbose error message at output checking

Author: cserteGT3

src/app/callbacks.jl

@@ -129,7 +129,7 @@ function _callback!(func::Union{Function, ClientsideFunction, String}, app::Dash
     check_callback(func, app, deps)
 
     out_symbol = Symbol(output_string(deps))
-    haskey(app.callbacks, out_symbol) && error("Multiple callbacks can not target the same output.")
+    haskey(app.callbacks, out_symbol) && error("Multiple callbacks can not target the same output. Offending output: $(out_symbol)")
     callback_func = make_callback_func!(app, func, deps)
     push!(
         app.callbacks,

test/callbacks.jl

@@ -319,9 +319,10 @@ end
         return value
     end
 
-    @test_throws ErrorException callback!(app, Output("my-div","children"), Input("my-id2","value")) do value
+    testresult = @test_throws ErrorException callback!(app, Output("my-div","children"), Input("my-id2","value")) do value
         return "v_$(value)"
     end
+    @test testresult.value.msg == "Multiple callbacks can not target the same output. Offending output: my-div.children"
 end
 
 @testset "empty triggered params" begin