restore --only fails

[sourcefrog]

In the current release it just says: Failed to restore file, which is very unhelpful, but improving the error message shows ERROR conserve: Failed to restore file "...": No such file or directory (os error 2)

I guess the problem is that --only isn't creating the subdirectories.

• Include source errors in the display for errors that have a source.
• Create subdirectories.
• Add a test for restore --only, or work out why it's not catching this.