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
`s/x/fieldMapper`