Vague error message when HOME is not set

[4z3]

Environment

• ejabberd version: 25.04
• Erlang version: Erlang (SMP,ASYNC_THREADS) (BEAM) emulator version 15.2.7
• OS: Linux (NixOS 25.05)
• Installed from: distro package

Errors from error.log

[critical] <0.117.0>@ejabberd_app:start/2:76 Failed to start ejabberd application: Exception occurred during configuration processing. This is most likely due to faulty/incompatible validator in third-party code. If you are not running any third-party code, please report the bug with ejabberd configuration file attached and the following stacktrace included:
** exception error: no match of right hand side value error
   in function  ejabberd_config:get_predefined_keywords/1 (/build/source/src/ejabberd_config.erl, line 506)
   in call from ejabberd_config:validators/3 (/build/source/src/ejabberd_config.erl, line 593)
   in call from ejabberd_config:'-validators/2-fun-0-'/4 (/build/source/src/ejabberd_config.erl, line 366)
   in call from lists:foldl/3 (lists.erl, line 2146)
   in call from ejabberd_config:validators/2 (/build/source/src/ejabberd_config.erl, line 364)
   in call from ejabberd_config:validate/1 (/build/source/src/ejabberd_config.erl, line 710)
   in call from ejabberd_config:read_file/2 (/build/source/src/ejabberd_config.erl, line 637)
   in call from ejabberd_config:load_file/1 (/build/source/src/ejabberd_config.erl, line 737)

Bug description

When attempting to run ejabberd without the HOME environment variable set, it fails with an error message that requires the user to consult the source code for clarification.

[badlop]

I've committed a fix: if HOME is not set, it uses the mnesia spool directory and logs an informative message.

There are binary installers available in https://github.com/processone/ejabberd/actions/runs/15705453621