Fix user preferences when deleting branch

When you load a project, and the target branch is not found (for example the user preferences point to a now deleted branch), the server will fall back to default branch (master) in the user preferences.
The client, when loading a project, will display a dialog box with a message explaining what happened, and will reload the project.
Note: when you are using the project without a reload, you will now get a clean "branch not found" dialog box, explaining which branch cannot be found on which project.

Using Studio with Branch Management:

User 1

• Creates a branch named "user1"
• Checks out branch "user1"
• Logs out

User 2

• Deletes branch "user1" while being on master branch

User 1

• Logs in
• Tries to load the project
  => and ends up with an error 500 that prevents the project from loading