Don't error when default config for toolkit settings is not present

Like https://github.com/pop-os/cosmic-settings-daemon/pull/100. We don't
appear to install default configs for these settings anywhere? And we
may not want libcosmic to inherently depend on globally installed config
files.

So just avoid printing errors in that case.

We may want to consider what errors `ConfigGet` returns
(https://github.com/pop-os/libcosmic/pull/948) but this should do for
now.

This should help with a big part of the log spam libcosmic is currently
producing.

src/app/cosmic.rs

@@ -369,6 +369,12 @@ where
                         .into_iter()
                         .filter(cosmic_config::Error::is_err)
                     {
+                        if let cosmic_config::Error::GetKey(_, err) = &why {
+                            if err.kind() == std::io::ErrorKind::NotFound {
+                                // No system default config installed; don't error
+                                continue;
+                            }
+                        }
                         tracing::error!(?why, "cosmic toolkit config update error");
                     }
 

src/config/mod.rs

@@ -25,6 +25,12 @@ pub static COSMIC_TK: LazyLock<RwLock<CosmicTk>> = LazyLock::new(|| {
             .map(|c| {
                 CosmicTk::get_entry(&c).unwrap_or_else(|(errors, mode)| {
                     for why in errors.into_iter().filter(cosmic_config::Error::is_err) {
+                        if let cosmic_config::Error::GetKey(_, err) = &why {
+                            if err.kind() == std::io::ErrorKind::NotFound {
+                                // No system default config installed; don't error
+                                continue;
+                            }
+                        }
                         tracing::error!(?why, "CosmicTk config entry error");
                     }
                     mode