rework validation

[christophsax]

improve error messages displayed in UI

• [x] 1. Remove dependencies on data for block constructors
• [x] 2. Get rid of the dual constructor setup, i.e. only have select_block() instead of also having new_select_block()
• [x] 3. Same for field constructors -> constructor should not do initialization/validation for us
• [x] 4. Start removing "fixing things" in the validator functions
• [x] 5. Evaluation stops as soon as block validation fails
  • eval stays in a try-catch
  • An error here has similar consequences as a validation failure
• [x] 6. UI/communication component