handle exception when removing a non existing box version

When trying to remove a not existing version of an existing box an
exception is thrown. The exception should be handled and some message
(box version 0.1.2 not available) should be printed.

With this patch the exception is handled correctly by using a newly
introduced exception BoxRemoveVersionNotFound.

Example output:

You requested to remove the box 'berendt/alpine-2.7.5-amd64' with provider
'virtualbox' in version '0.0.2'. The box 'berendt/alpine-2.7.5-amd64'
exists but not in the specified version '0.0.2'.
Please double-check and try again.

The available versions for this box are: 0.1.0

Fixes issue #3344