use errors.Is() to check for errors (#1730)

Since go 1.13 you can wrap errors. This make it no longer possible to
compare with `==`, instead you have to compare with `errors.Is()`.

I noticed this problem because -h was no longer working after I stared
wrapping the errors in my custom FlagErrorFunc function.
Note that this is only a problem when a custom help flag is defined.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>