Update btstep negative eta warning

[awallcraft]

In Boussinesq mode the eta below bathyT warning includes the location, but in non-Boussinesq mode the corresponding negative eta warning does not.

Added the location to the negative eta warning.

No answers are changed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 21.69%. Comparing base (5e4f97b) to head (e13b8f7).

❗ There is a different number of reports uploaded between BASE (5e4f97b) and HEAD (e13b8f7). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (5e4f97b)	HEAD (e13b8f7)
	2	1

Additional details and impacted files

@@              Coverage Diff              @@
##           dev/gfdl     #844       +/-   ##
=============================================
- Coverage     38.10%   21.69%   -16.41%     
=============================================
  Files           298      137      -161     
  Lines         87711    33202    -54509     
  Branches      16447     5951    -10496     
=============================================
- Hits          33418     7202    -26216     
+ Misses        48264    25443    -22821     
+ Partials       6029      557     -5472     

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