pbx_lua: On configuration errors report module load failure instead of decline.

Switched from AST_MODULE_LOAD_DECLINE to AST_MODULE_LOAD_FAILURE.
Therefore, if pbx_lua fails to load and pbx_lua is marked as required,
Asterisk exits as expected.
If extensions.lua cannot be opened, AST_MODULE_LOAD_DECLINE is reported.

Change-Id: I8e5a0037e69b41743db60c568541ebb2f52a7a8f