Improve error message if Log4j API implementation missing

[ppkarwasz]

As showed by this SO question 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.