No error when purging sticky modules

[xdelaruelle]

Currently an error is raised when doing a module purge if some sticky modules are loaded.

This is done as purge should result in an empty loaded module list.

But some users may find this error disturbing as sticky module cannot be unloaded, so purge has performed well.

A configuration option may be introduced to change error into warning (or even no message at all) when hitting a sticky module during a module purge.

[adrien-cotte]

I agree this option should have 3 options:

• error (message + code 1)
• warning (message + code 0)
• silent (no message + code 0)