Improve err reporting when filter-netns cannot be found

Previously:

    $ pwru --filter-netns /proc/self/ns/netns
    2023/09/28 14:58:35 Failed to get pwru config: no such file or directory

After this change:

    $ pwru --filter-netns /proc/self/ns/netns
    2023/09/28 14:59:36 Failed to get pwru config: Failed to retrieve netns /proc/self/ns/netns: no such file or directory

Signed-off-by: Martynas Pumputis <m@lambda.lt>

internal/pwru/config.go

@@ -57,6 +57,7 @@ func GetConfig(flags *Flags) (cfg FilterCfg, err error) {
 
 	netnsID, ns, err := parseNetns(flags.FilterNetns)
 	if err != nil {
+		err = fmt.Errorf("Failed to retrieve netns %s: %w", flags.FilterNetns, err)
 		return
 	}
 	if flags.FilterIfname != "" || flags.FilterNetns != "" {