i2c: improve error messages in i2c_register_adapter()

Switch to WARN if no adapter name is given, otherwise we won't know who
missed to do that. Add error message if device registration fails.
Update error message for missing algo to match style of the others.

Signed-off-by: Wolfram Sang <wsa-dev@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>