Incorrect exception when requested menu does not exist.

[dantleech]

The exception is thrown as follows:

The menu root "/cms/contents" doe not exist.

When the menu root does exist but the requested menu doesn't.

[ElectricMaxxx]

Is that relevant for 2.0 (CMF) or gone through the admin extension?

(PR is marked with 2.0)

[wouterj]

This is not related to sonata at all, so still relevant.