[configuration] print better error messages on loading

[mikz]

fixes https://issues.jboss.org/browse/THREESCALE-80

• print reason why configuration could not be downloaded
• print more debugging information

example log:

2017/03/16 08:08:15 [error] 30795#0: [lua] configuration_loader.lua:80: init(): boot could not get configuration (exit 1)
2017/03/16 08:08:16 [warn] 30816#0: *2 [lua] remote_v2.lua:58: call(): failed to get list of services: invalid status, context: ngx.timer
./src/configuration_loader.lua:25: missing configuration

2017/03/16 08:08:15 [emerg] 30795#0: [lua] configuration_loader.lua:101: init(): failed to load configuration, exiting