CMIP7: processing of grid-W files triggers depth coord Error that does not get reported back to MIP Convert

in the cmor logs for grid-W processing I can see errors such as

```
ESC[2;31;47mC Traceback:
! In function: cmor_treat_axis_values
! called from: cmor_axis
! ESC[0m

ESC[1;31;47m!!!!!!!!!!!!!!!!!!!!!!!!!
!
! Error: axis depth_coord (table: ocean), detected value at: -0.505760 when
! valid_min is 0.000000
!
!!!!!!!!!!!!!!!!!!!!!!!!!ESC[0m

```
which is not being flagged up to CDDS as a failure in CMOR. This is likely a CMOR issue. The processing in question does produce data with bounds values below zero, which may not be correct.

I'm not too concerned about this, but we should fix in the near future.

Start by reproducing grid-W processing and tracing the cmor_treat_axis_values/cmor_axis error path shown in the CMOR logs through CDDS reporting. Done means the depth_coord validation failure is surfaced to MIP Convert and the handling of bounds values below zero is addressed or clearly resolved.

Written by the indexing model from the issue text.