Cryptic errors in some situations

[dpordomingo]

related to #486

There are some situations that cause unhandled errors like the following one.

[2019-01-23T11:17:34.461935361+01:00] ERROR event processing failed 
app=lookoutd 
error=posting analysis failed: %!s(PANIC=runtime error: invalid memory address or nil pointer dereference) 
event-id=b44c158d09bebb40b85ce23763dd9ab179926019 
event-type=*pb.ReviewEvent 
github.pr=63 
head=refs/pull/63/head 
repo=https://github.com/dpordomingo/testing-repo.git

When it happens, it's difficult to know what's causing the error.
In this case, I found that it was related to google/go-github#540

It would be a good idea to provide more details in those logs.