Errors when running bitcoind at startup
The same problem occured on my Ubuntu 16.04 with Bitcoin v0.15.1.

After some investigation, I found the reason is that the file(init.md) is out of date. Ubuntu has used systemd instread of upstart since 15.10, while the section of ubuntu in that file don't update in time.

It's sure that the lastest modification is 2015-08-04, which is before the release date of Ubuntu 16.04.


$ git blame -L94,97 doc/init.md
234bfbf6 (Adam Weiss   2014-07-31 11:56:17 -0400 94) 4c) Upstart (for Debian/Ubuntu based distributions)
234bfbf6 (Adam Weiss   2014-07-31 11:56:17 -0400 95)
fc25a874 (unsystemizer 2015-08-04 14:10:51 +0800 96) Drop bitcoind.conf in /etc/init.  Test by running `service bitcoind start`
234bfbf6 (Adam Weiss   2014-07-31 11:56:17 -0400 97) it will automatically start on reboot.


The section, "4a) systemd", is valid, which works on me.



Reference


What's New in Ubuntu 16.04 : The systemd Init System
How to identify which init system you are currently booting with