Improve error message if Log4j API implementation missing

As showed by [this SO question](https://stackoverflow.com/q/79389054/11748454) users might be quite surprised by the error message printed, when no Log4j API implementation is present (or enabled) on the classpath.

The message should probably be similar to the one printed by SLF4J:

```
SLF4J: No SLF4J providers were found.
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#noProviders for further details.
```

and point to our [Installation instructions](https://logging.apache.org/log4j/2.x/manual/installation.html#impl).

Start by locating the error path used when no Log4j API implementation is present or enabled on the classpath. Compare its output with the example SLF4J messages and update the guidance to point to the Installation instructions at logging.apache.org/log4j/2.x/manual/installation.html#impl; done means users receive a clear explanation and setup link.

Written by the indexing model from the issue text.