[GRASS-dev] Error message for grass MAPSET --exec

[Nikos Alexandris]

If an extension is not installed, then

```
grass72 /geo/grassdb/global/mdt_wgs84/PERMANENT/ --exec i.landsat8.swlst
Cleaning up temporary files...
Starting GRASS GIS...
Executing <i.landsat8.swlst> ...
```

should report that the extension is not installed, instead of

```
ERROR: Execution of <i.landsat8.swlst> failed:
[Errno 2] No such file or directory
Exiting...
```

which can/does make a user think there is missing/potentially faulty input file,
subject to an extension's parameter.

Makes sense?
Thank you, Nikos
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