[Tile-serving] [openstreetmap/osm2pgsql] Confusing error when database parameter is missing (#1400)

[Sarah Hoffmann]

When running osm2pgsql without naming a database via `-d` or PGDATABASE, then the version check prints a rather confusing message:

```
me at machine:~$ ./osm2pgsql -S ../default.style ~/osm/data/liechtenstein-latest.osm.pbf 
2021-01-29 17:54:41  osm2pgsql version 1.4.0 (1.4.0-82-gfd9c0c33)
2021-01-29 17:54:41  Database version: 11.9 (Debian 11.9-0+deb10u1)
2021-01-29 17:54:41  ERROR: Database error: ERROR:  function postgis_lib_version() does not exist
LINE 1: SELECT regexp_split_to_table(postgis_lib_version(), '\.')
                                     ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
```

The same happens when the database is right but the postgis extension is missing. We should print a more helpful message here.

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