More explicit message when workspace configuration file doesn't exist

Summary:
An exception is thrown by Workspace::loadConfiguration if the workspace configuration file doesn't exist.

This exception weren't intercepted. We offer an interception and more explicit error messages.

New l10 keys:

• CantLoadWorkspace, title of the error message if there is an issue getting workspace information
• NotConfiguredWorkspace, content of the error message if the configuration file doesn't exist

Fixes T464.

Test Plan:

• Added a new workspace quux in database, but not in assets file
• Got the appropriate error message on http://obsidian51.trantorium.com/quux

Reviewers: dereckson

Maniphest Tasks: T464

Differential Revision: http://devcentral.nasqueron.org/D15