Suggest -debug option if connection to pipe failed

[vient]

I spent an hour right now trying to debug error connecting to pipe on test run without afl-fuzz. It occured that I forgot the -debug option. While the whole thing is kinda obvious, if you need test run then you shouldn't forget the -debug, a suggestion about it on pipe error would've been very helpful.

What I propose is to edit error message so it would be error connecting to pipe (did you forgot the -debug option?) or something similar.