Send debug output to stderr when copying certs
Check all necessary environment variables

If any of DOCKER_HOST, DOCKER_CERT_PATH or DOCKER_TLS_VERIFY are
incorrect, `boot2docker up` will inform the user, and
`boot2docker shellinit` will output the correct commands to set/unset
them.

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>