Fixing name collision of driver.ErrMachineNotExist and virtualbox.ErrMachineNotExist
Moving ErrMachineExist to driver package
"machine already exists" is not virtualbox specific
Fixing error message when no suitable driver found

Right now The error message is misleading in case of a non existing driver: "boot2docker-vm": machine not supported
Its not the machine which is not supported, but the driver

    $ boot2docker --driver=nosuchdriver info

    Boot2Docker-cli version: v1.3.0
    Git commit: 1d0b71b
    error in run: Failed to get machine "boot2docker-vm": machine not supported
Merge pull request #290 from lalyos/error-handling

Fixing some error handling issues