Confusing error when postigs extension is missing

[lonvia]

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

me@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.

[pnorman]

What database is it connecting to?

[stephankn]

Do I understand correctly, that the request is to first check whether the postgis extension is installed, before calling postgis functions?

Like:

postgres=# SELECT count(*) FROM pg_extension WHERE extname='postgis';
 count
-------
     1
(1 row)

And if not printing an error message?

[lonvia]

What database is it connecting to?

It was connecting to a database under my username, which happened to exist. Issue title fixed.