Okapi error on export is not correctly handled in translate5

If the Okapi conversion on export fails, no error is communicated to the user. What should happen:

• the error should be logged with the task
• the file name of the export-zip should get the prefix "ERROR_ON_EXPORT_DETAILS_IN_ZIP"

In addition the content of the xliff is not saved as xlf-file in the exported zip folder, but in the file name of the original file format (so you get e.g. a docx-file with xlf-content). Instead, the xliff still should be saved as xlf in the zip and the original file should simply be left out.

For example data please use the data attached to the linked TS issue. Additional example data can be found in the task, that is described in the linked TS issue.

Another related problem is, that if a file (for example a SVG file) can not be imported due Okapi import errors, it will remain as file in the file list. Therefore on the export an fatal error about a not found PHP class is thrown and the task is set to state error. The problem is, that no fileparser is registered for such a file with failed okapi import. As solution we produce a dedicated warning if no fileparser is registered for a file, but the other files can be exported and the task remains usable.