Handle messages about duplicated files in ESR

[metas-rc]

Followup for #142

Is this a bug or feature request?

• FR

What is the current behavior?

• When trying to reimport a file in ESR using webui, there is always an error "File not imported"

Which are the steps to reproduce?

• Try and see

What is the expected or desired behavior?

• In case of trying to import the same file again, I get " File not imported"error.
• The reason for this bug is that our warnings with yes/no confirmations and our other swing messages are not supported by webui.
• The sys config does not allow any options of ignoring the fact that a file is duplicated.
  We have only these 2 valid values: W - Warning with buttons Yes and No, E -Entirely, Warning with OK button.
• How shall we proceede with this part in webui?
• Stacktrace provided in this pastebin: https://pastebin.com/8qJ3gNKU