Don't show an error when cancelling an export

If the file type prompts you before actually saving (for example, PNG) it's still possible to cancel the export process. However the previous logic considers this a failure, and will show a pretty unhelpful dialog that there was an "unknown failure" while exporting your document.

Instead, lets treat it like a cancelled export which doesn't display a dialog. startExportInBackground unfortunately doesn't provide enough information to the callee since a boolean can only state if it was successful or not, I chose to just bubble up the KisImageExportErrorCode instead.

BUG: 501128

Test Plan

1. Open a document
2. Export a document with a file type that prompts before saving (PNG is an easy one)
3. Cancel from the export dialog
4. Confirm that no error dialog is shown (for cancelling, normal errors should still show)

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!) (I ran TestDocument, I'm not sure if there's others relevant for export)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.