Fix async export

[BartChris]

resolves #5481
resolves #5008

This pull request tries to adress the problems outlined in the above issue by

• not immediatly indicating a succesful export in case of an asynchronous export and prevent closing a workflow step after a failed automatic export
• ensuring that the export status is correctly propagated through all the method calls involved
• removing the static "export finished" message from the Desktop and Process list form to ensure that a success message is only displayed in case of an actual succesful export

[solth]

@BartChris thanks for the pull request. Can you please rebase your branch against the current master to resolve the build issue caused by an outdated chrome driver?

[solth]

@BartChris please rebase against current master since your PR is the next merge candidate