Improved logging

[kpcyrd]

Instead of returning a string with the build log this now creates a Vec<u8> and passes it as &mut Vec<u8>, in case of any error (even after we've already logged output from the child process) we append to this Vec too and send it back to the daemon, so the build log up to that point isn't lost.

We also include the following info in logs now:

• the size limit, if reached
• the time limit, if reached
• the human readable error, if any, instead of just rebuilderd: unexpected error while rebuilding package