fix(balancer) avoid crash when failing to obtain list of upstreams

[hishamhm]

Avoid a stacktrace when the database fails to obtain the list of upstreams (as
seen in an error log provided by @hutchic): instead report a CRIT-level
message and fallback to an empty list.

2019/02/12 17:38:18 [crit] 36#0: *7 [lua] balancer.lua:719: init(): failed loading initial list of upstreams: failed to get from node cache: callback threw an error: /usr/local/share/lua/5.1/kong/runloop/balancer.lua:476: attempt to index local 'up' (a
boolean value)
stack traceback:
       /usr/local/share/lua/5.1/kong/runloop/balancer.lua:476: in function </usr/local/share/lua/5.1/kong/runloop/balancer.lua:472>
       [C]: in function 'xpcall'
       /usr/local/share/lua/5.1/resty/mlcache.lua:766: in function 'get'
       /usr/local/share/lua/5.1/kong/cache.lua:126: in function 'get'
       /usr/local/share/lua/5.1/kong/runloop/balancer.lua:489: in function 'get_all_upstreams'
       /usr/local/share/lua/5.1/kong/runloop/balancer.lua:717: in function 'init'
       /usr/local/share/lua/5.1/kong/runloop/handler.lua:565: in function </usr/local/share/lua/5.1/kong/runloop/handler.lua:564>, context: ngx.timer