Fix bugs in CipherStash::Client.login

* A default profile would not be created correctly if a workspace was given but a profile was
  not passed to `Client.login`. Always providing a profile name fixes that.
* If neither a workspace or profile name were given and a profile could not be loaded, an
  unhelpful auth error would be raised. Instead, explicitly raise that a profile could not
  be loaded (by always providing a profile name to `Profile#load`) so that downstream libs
  (ActiveStash) can handle this scenario properly.

Author: CDThomas

lib/cipherstash/client/profile.rb

@@ -128,7 +128,7 @@ def self.create(name, logger, **opts)
 
       def self.login(workspace:, profile_name:, logger:)
         is_initial_login = !workspace.nil?
-        profile_name = resolve_profile_name(profile_name)
+        profile_name = resolve_profile_name(profile_name) || "default"
 
         if is_initial_login
           create(profile_name, logger, workspace: workspace)