g.download.location: Print target path in error message (OSGeo#4155)

* g.download.location: print target path in error message

To be less obscure in case the target location directory already exists:

```
ERROR: Location named <nc_spm_full_v2alpha2> already exists, download
       canceled
```

this PR improves the error message to include the path:

```
g.download.location url=https://grass.osgeo.org/sampledata/north_carolina/nc_spm_full_v2alpha2.tar.gz path=$HOME
ERROR: Location named <nc_spm_full_v2alpha2> already exists in
       </home/mneteler/nc_spm_full_v2alpha2>, download canceled
```

This is esp. relevant for scripted usage when the command isn't invoked directly by the user.

In addition, section  of **EXAMPLES** added.

* fix message format

Co-authored-by: Vaclav Petras <wenzeslaus@gmail.com>

* database -> project

Co-authored-by: Veronica Andreo <veroandreo@gmail.com>

---------

Co-authored-by: Vaclav Petras <wenzeslaus@gmail.com>
Co-authored-by: Veronica Andreo <veroandreo@gmail.com>

scripts/g.download.location/g.download.location.html

@@ -11,6 +11,27 @@ <h2>DESCRIPTION</h2>
 The first directory which is a project is used.
 Other projects or any other files are ignored.
 
+<h2>EXAMPLES</h2>
+
+<h3>Download the full GRASS GIS sample project within a running session</h3>
+
+Download and unpack the full North Carolina sample project into the user's
+HOME directory:
+
+<div class="code"><pre>
+g.download.location url=https://grass.osgeo.org/sampledata/north_carolina/nc_spm_full_v2alpha2.tar.gz path=$HOME
+</pre></div>
+
+<h3>Download the full GRASS GIS sample project in a temporary session</h3>
+
+In a temporary session, download and unpack the full North Carolina sample project
+into the user's HOME directory:
+
+<div class="code"><pre>
+grass --tmp-location XY --exec g.download.location url=https://grass.osgeo.org/sampledata/north_carolina/nc_spm_full_v2alpha2.tar.gz path=$HOME
+</pre></div>
+
+
 <h2>SEE ALSO</h2>
 
 <em>

scripts/g.download.location/g.download.location.py

@@ -102,7 +102,9 @@ def main(options, unused_flags):
 
     if destination.exists():
         gs.fatal(
-            _("Location named <{}> already exists, download canceled").format(name)
+            _(
+                "Location named <{name}> already exists in <{directory}>, download canceled"
+            ).format(name=name, directory=database)
         )
 
     gs.message(_("Downloading and extracting..."))