SingularExceptions more common, add to list of common errors in docs, and insert better error message

## Issue
The default localizer in EKI `NICE(1,1)` is sometimes overly penalizing the ensemble (e.g. where the ensemble size is already larger than the effective dimension of the problem).

This causes unhelpful SingularException errors

## Simplest solution
- Reducing the first parameter of the localizer to something small by default e.g. `NICE(0.05,1)` (effectively reducing the localizer strength) can ensure more robustness in general.
- Improving the error message to indicate that if `NICE(0.05,1)` is active, then the user can choose to reduce the values.

No files or tests are named. Start by locating the default localizer configuration and the code path that raises SingularException, then find the documentation section listing common errors. Done means the default behavior and error message are improved as described, the SingularException guidance is documented, and the relevant tests pass.

Written by the indexing model from the issue text.