Improve error reporting

[RamonMeffert]

Instead of the error being just a string, make it a record so the rendering can be improved. Something like:

type alias Error =
    { title : String
    , message : String
    , cause : String
    , advice : String
    }

[RamonMeffert]

Should also have a severity field

[RamonMeffert]

• Also, a 'global' method to render these errors is needed as well

[RamonMeffert]

Note: errors are pretty neat already, it's just that they could be even better

[RamonMeffert]

Errors are good enough generally, but there are a few cases that could use improvement:

• A complex input string for gossip graphs has an error. This will generate an error like "Agent X is unknown", leaving the user to manually search for the error in the input string. If possible, it would be nice to show a message like the following:

  Abc aBc abX
            ↑
          here
  
  I do not know agent X! Maybe you meant to include an extra agent, or you made a typo?
  

  I already have the position of the error in the string, so this would (/should) not be too difficult. Will of course only work when using a monospace font.

• The same issue as above, but for call sequences

• Maybe more?