Improve warning & error messages

[rvcas]

• Better warnings for unused fields

  The help for unused record field gives instructions that are useful for unused let bindings but it could show the user how to ignore a field with either

  .. or y: _thing

  

• Better parse error for incomplete / invalid records. See also Aiken check/parser innacurate pointing on custom data type #650.

• Allow accumulating more than one error before exiting. (extracted from Allow accumulating more than one error before exiting #824)

  Specifically in the context of type checking and while running the language server (aiken lsp), it would
  be nice if the more than one error could be show at a time in an editor. In the command line things would stay as is. This is useful when getting feedback in an editor that supports LSP. We totally can make it happen especially
  for exhaustiveness errors.