Add parameter to GET for checking if generated fields can be retrieved

Fields of type `token_count`, `murmur3`, `_all` and `_field_names` are generated only when indexing.
If a GET requests accesses the transaction log (because no refresh
between indexing and GET request) then these fields cannot be retrieved at all.
Before the behavior was so:

`_all, _field_names`: The field was siletly ignored
`murmur3, token_count`: `NumberFormatException` because GET tried to parse the values from the source.

In addition, if these fields were not stored, the same behavior occured if the fields were
retrieved with GET after a `refresh()` because here also the source was used to get the fields.

Now, GET accepts a parameter `ignore_errors_on_generated_fields` which has
the following effect:
- Throw exception with meaningful error message explaining the problem if set to false (default)
- Ignore the field if set to true
- Always ignore the field if it was not set to stored

This changes the behavior for `_all` and `_field_names` as now an Exception is thrown if a user
tries to GET them before a `refresh()`.

closes #6676
maybe instead of passing a boolean to the abstract field mapper we can just return `false` from here and the exceptional mappers that implement that can just override it and return true? that way we don't break compat with others that implement mappers and the change is less intrusive?