more concise message for BoxNotFound error

Now we have different providers, but the error message didn't tell
anything about it. Suppose I want to remove one of my boxes:

    vagrant box remove opscode-ubuntu-12.04 vritualbox

There is a typo in provider name. The error message is:

    Box 'opscode-ubuntu-12.04' could not be found.

Therefore I need to double check the box name, and only than I will see
the typo.

This commit make the error message looks like this:

    Box 'opscode-ubuntu-12.04' with 'vritualbox' provider could not be found.