Malformed recentworkspaces.json can crash startup

[colin-grant-work]

Bug Description:

We use a file called recentworkspaces.json to track the user's recent workspaces so that we can populate the 'recent workspaces' list and restore the workspace used in the last session. If that file contains valid JSON but is in some way malformed, it can crash startup. In general, we shouldn't have to worry much, because users shouldn't touch that file (In my case, making changes to the JSONRPC system seems to have caused a bad write), but it might be good to make the code that handles it more robust so that we can startup when it's a bad state, and perhaps clear or repair it.

Steps to Reproduce:

1. Open your recentworkspaces.json file (default location is ~/.theia/recentworkspaces.json)
2. It should be in the form {"recentRoots": ["root1", "root2", ...]}
3. Modify it so that it has the form {"recentRoots": [["root1"], "root2"]}
4. Start the application (I was using Electron)
5. The application should fail to start, and the backend logs should have a message about missing fsPath of undefined.

Additional Information

• Operating System: RHEL7, Electron
• Theia Version: master