Closes #4464

Before:

After (Origine):

After (Mapco):

After (Mapco):

Changes proposed in this pull request:

• add a triangle symbol next to the feed name in sidebar and in subscription management
• give a mouse over text, when an error occured

How to test the feature manually:

1. find a feed with an error
2. mouse over it (see the text)
3. see the red triangle

Pull request checklist:

• clear commit messages
• code manually tested