chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/cli/formatservers.go

@@ -16,11 +16,6 @@ import (
 	"golang.org/x/text/language"
 )
 
-var (
-	ErrProviderUnspecified       = errors.New("VPN provider to format was not specified")
-	ErrMultipleProvidersToFormat = errors.New("more than one VPN provider to format were specified")
-)
-
 func addProviderFlag(flagSet *flag.FlagSet, providerToFormat map[string]*bool,
 	provider string, titleCaser cases.Caser,
 ) {
@@ -65,11 +60,10 @@ func (c *CLI) FormatServers(args []string) error {
 	}
 	switch len(providers) {
 	case 0:
-		return fmt.Errorf("%w", ErrProviderUnspecified)
+		return errors.New("VPN provider to format was not specified")
 	case 1:
 	default:
-		return fmt.Errorf("%w: %d specified: %s",
-			ErrMultipleProvidersToFormat, len(providers),
+		return fmt.Errorf("more than one VPN provider to format were specified: %d specified: %s", len(providers),
 			strings.Join(providers, ", "))
 	}