loader: Flag module as declined in all cases where it fails to load.

This has no effect on startup since AST_MODULE_LOAD_FAILURE aborts
startup, but it's possible for this code to be returned on manual load
of a module after startup.

It is an error for a module to not have a load callback but this is not
a fatal system error.  In this case flag the module as declined, return
AST_MODULE_LOAD_FAILURE only if a required module is broken.

Expand doxygen documentation for AST_MODULE_LOAD_*.

Change-Id: I3c030bb917f6e5a0dfd9d91491a4661b348cabf8