Detect JSON issue parsing workspace configuration file.

Summary:
If the workspace configuration file has a JSON issue, we detect it and print
the result of json_last_error_message() to help the admin to solve the issue.

Fixes T465.

Test Plan:

• fortune > workspaces/quux/workspace.conf
• Got the error message on http://obsidian51.trantorium.com/quux

Reviewers: dereckson

Maniphest Tasks: T465

Differential Revision: http://devcentral.nasqueron.org/D16