Invalid CSS breaks the build

[Stvad]

Describe the bug
When user uploads invalid CSS that breaks the site build and does not notify (#16 ) user about the failure which is a pretty bad experience

Expected behavior
In the order of preference:

• Input validation during upload reports a problem to the user immediately
• Build succeeds but css is not applied
• If build fails - user gets notification Notification on site build success/failure #16 with explanation