Add back compat for volume drivers Get and Ls

Use a back-compat struct to handle listing volumes for volumes we know
about (because, presumably, they are being used by a container) for
volume drivers which don't yet support `List`.

Adds a fall-back for the volume driver `Get` call, which will use
`Create` when the driver returns a `404` for `Get`. The old behavior was
to always use `Create` to get a volume reference.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

plugins/errors.go

@@ -0,0 +1,33 @@
+package plugins
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type statusError struct {
+	status int
+	method string
+	err    string
+}
+
+// Error returns a formated string for this error type
+func (e *statusError) Error() string {
+	return fmt.Sprintf("%s: %v", e.method, e.err)
+}
+
+// IsNotFound indicates if the passed in error is from an http.StatusNotFound from the plugin
+func IsNotFound(err error) bool {
+	return isStatusError(err, http.StatusNotFound)
+}
+
+func isStatusError(err error, status int) bool {
+	if err == nil {
+		return false
+	}
+	e, ok := err.(*statusError)
+	if !ok {
+		return false
+	}
+	return e.status == status
+}