xdg: move `cola.xdg.config_home()` to `cola.resources`

Signed-off-by: David Aguilar <davvid@gmail.com>
cmds: move the UsageError class to cmds

Elminate the `cola.errors` module entirely by moving the UsageError
exception alongside its caller.

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: v1.9.2 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>