g.download.project: Fix string formatting error when download already exists

[echoix]

When trying to test locally for #5338, I stumbled across an error on Windows, but couldn't see it. The translated string contained both named placeholders and empty placeholder, but used keyword arguments. I don't know why I never noticed on Linux, but on Windows, there was no indication of what was happening.

d:\repos\grass_repo\grass\vector\v.univar>grass85 --tmp-project XY --exec g.download.project url=https://grass.osgeo.org/sampledata/north_carolina/nc_spm_full_v2alpha2.tar.gz path=%USERPROFILE%
WARNING: Concurrent mapset locking is not supported on Windows
Traceback (most recent call last):
  File "C:\OSGeo4W\apps\grass\grass85/scripts/g.download.project.py", line 153, in <module>
    main(*gs.parser())
  File "C:\OSGeo4W\apps\grass\grass85/scripts/g.download.project.py", line 107, in main
    ).format(name=name, directory=database)
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IndexError: Replacement index 0 out of range for positional args tuple
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After this fix:

d:\repos\grass_repo\grass\vector\v.univar>grass85 --tmp-project XY --exec g.download.project url=https://grass.osgeo.org/sampledata/north_carolina/nc_spm_full_v2alpha2.tar.gz path=%USERPROFILE%
WARNING: Concurrent mapset locking is not supported on Windows
ERROR: Project named <nc_spm_full_v2alpha2> already exists in
       <C:\Users\E-C>, download canceled
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[echoix]

I've skimmed through the 2789 results in python files for {} (grass and grass-addons), and I didn't find any other similar cases. I've also searched for the regex \{\}.*(.|[\n])+?\.format\(.*=.*\), and didn't find any similar case (that regex was able to find the error from this PR, when it wasn't a false positive match).