Summary: If programs would have useful man pages or error messages, man would not have to look into source code.
I just set up Debian sid chroot on my computer and have to sign something inside this chroot (dpgk-buildpackage). Okay, I want it to use gpg-agent in my host system, that should not be hard. Looking at gpg man page, GPG_AGENT_INFO variable is used to forward information about agent socket, PID and protocol version, so I set this variable according to host system, but it still does not work, giving nothing saying error that agent is not working.
So let's debug this beast - strace reveals, that communication with agent works, but it replies "113 Server Resource Problem". Hmm, still not much useful information, but let's google for this problem. Fortunately Google reveals related Seahorse code, where man can find, that not set DISPLAY variable is causing this issue. Could not the error be more friendly, so that it tells me directly what is wrong?