client: some improvements for Client.cc

client: some improvements for Client.cc

1. Log entry is too short and multiple member functions can have same
directory and therefore it's best to prefix log entry with member
function's name so that it is easy to locate it in codebase.

2. Variable name "err" doesn't necessarily hold error code but may hold
a generic return value as well. Renaming it to "retval" will mildly and
quietly emphasize this to the reader.

3. Fix log entry in _rmdir() which incorrectly prefixes itself with "rmdir".

Signed-off-by: Rishabh Dave <ridave@redhat.com>