Allow empty descriptions for reuploads

There is no need for descriptions if an existing file is overwritten.
Therefore the check and the error message are moved at a point where we
know whether it is an initial upload or a reupload.

- refactor MediaWikiInterface.uploadFile:
- add the parameter hasDescription that describes whether there is a
valid description
- return a string (error message) if there is an error
- store description status in
MediaWikiExportServiceProvider.prepareUpload
- check the return value of MediaWikiInterface.uploadFile in
MediaWikiExportServiceProvider.prepareUpload

Fix https://github.com/ireas/LrMediaWiki/issues/30