More explicit errors on TLS failure.

When getting an HTTP connection error different than connection refused,
issue a warning about connecting to TLS enabled daemons with no
TLSConfig, similar to what the Docker CLI is doing.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>
Merge pull request #54 from aluzzardi/tls-config-check

More explicit errors on TLS failure.