Explicitly list invalid metadata keys

[Xpirix]

• This is a proposed fix for Explicitly list all the missing keys related to upload error #379

Change summary:

• List all missing metadata keys in the upload error (1st screenshot)
• List metadata keys for invalid URL link in the upload error (2nd screenshot)
• List metadata keys for inaccessible URL link in the upload error (3rd screenshot)
• Update unit test

ToDo:

When this is deployed, we should update the following line in https://plugins.qgis.org/publish/ :

The plugin metadata contains a link to the source code, an issue tracker and a license

To

The plugin metadata contains a valid link to the homepage, the repository (source code), the tracker (issue tracker) and a license