Bugfix subbasin

[hboisgon]

Issue addressed

Fixes #236
Fixes #261

Explanation

Earlier error messages.

Checklist

• Updated tests or added new tests
• Branch is up to date with main
• Tests & pre-commit hooks pass
• Updated documentation if needed

Additional Notes (optional)

Add any additional notes or information that may be helpful.

[shartgring]

Approved and code is good, but do check the comments on the description of the errors

[shartgring]

Looks good! Code works, still some changes are needed related to the error messaging

Also checked on local machine for following scenarios:

• build regular model, looks good
• too coarse resolution -> error as expected
• too high upa -> error as expected
• weird location for subbasin outlet -> error as expected
• missing streamflow info so too small model -> error as expected
• small model -> warning as expected