Added Error library for developer debugging

Summary:
Created a simple Phabricator version of slog called 'plog' that allows
developers to log debug information to Dark Console.

Test Plan:
Called 'plog' function from various places in Phabricator and checked that
the debug information was available in DarkConsole.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, epriestley, tuomaspelkonen
Differential Revision: 100