Do not error on installing duplicate shutdown handler

Installing a duplicate shutdown handler fails, but if a handler
with the same name is already present, we should be set to go.
There's no reason to print a user-facing error about it.

This comes up almost nowhere because Podman never makes more than
one Libpod runtime, but there is one exception (`system reset`)
and the error messages, while harmless, were making people very
confused (we got several bug reports that `system reset` was
nonfunctional).

Signed-off-by: Matthew Heon <mheon@redhat.com>

libpod/shutdown/handler.go

@@ -10,6 +10,10 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+var (
+	ErrHandlerExists error = errors.New("handler with given name already exists")
+)
+
 var (
 	stopped    bool
 	sigChan    chan os.Signal
@@ -98,7 +102,7 @@ func Register(name string, handler func(os.Signal) error) error {
 	}
 
 	if _, ok := handlers[name]; ok {
-		return errors.Errorf("handler with name %s already exists", name)
+		return ErrHandlerExists
 	}
 
 	handlers[name] = handler