Clarify log messages for configuration cache

When the cache file is not created yet, libcouchbase outputs message on the ERROR log level

Couldn't open for reading: No such file or directory

This message does not tell what is going on next. In fact, the libcouchbase will just continue to next configuration provider, loads the config, which in turn creates the cache file.

Another not so obvious error message is

[cb,EROR] (bc_file L:127 I:1313592241) (cache=/tmp/cb_config_cache) Bucket name is nullptr

Here what libcouchbase means is that it found the configuration cache, but the current lcb_INSTANCE does not have a bucket name associated yet, and it cannot apply the configuration cache.