Add explicit OK output to check command

This adds additional output to the check command when no errors were
found. It means that when all checks have been completed, the following
output is displayed:

	No errors were found

The output is added to make sure that it is easier to understand that no
errors were found.

Full example output:

	Create exclusive lock for repository
	Load indexes
	Check all packs
	Check snapshots, trees and blobs
	No errors were found

cmd/restic/cmd_check.go

@@ -176,5 +176,8 @@ func runCheck(opts CheckOptions, gopts GlobalOptions, args []string) error {
 	if errorsFound {
 		return errors.Fatal("repository contains errors")
 	}
+
+	Verbosef("No errors were found\n")
+
 	return nil
 }