doc: error_code_conventions -> conventions.rst

It was too short a file, it was causing duplicate link warnings
(_error_code_conventions, due to the inclusion, which wasn't being
used anyway).

This removes the littering of small files and removes warnings from
the documentation compile.

Change-Id: Ic6f225a63d875d77bd2e93b2712baabea2eb0141
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>