plugin/store.Get: return a specific error if plugin is disabled

Previously, a 'plugin not found' error would be returned if a plugin to be
retrieved was found but disabled. This was misleading and incorrect. Now,
a new error plugin.ErrDisabled is returned in this case. This makes the
error message when trying to statically start plugins (from daemon.json or
dockerd command line) accurate.

Signed-off-by: David Sheets <dsheets@docker.com>

plugin/store.go

@@ -36,6 +36,13 @@ func (name ErrAmbiguous) Error() string {
 	return fmt.Sprintf("multiple plugins found for %q", string(name))
 }
 
+// ErrDisabled indicates that a plugin was found but it is disabled
+type ErrDisabled string
+
+func (name ErrDisabled) Error() string {
+	return fmt.Sprintf("plugin %s found but disabled", string(name))
+}
+
 // GetV2Plugin retrieves a plugin by name, id or partial ID.
 func (ps *Store) GetV2Plugin(refOrID string) (*v2.Plugin, error) {
 	ps.RLock()
@@ -138,7 +145,7 @@ func (ps *Store) Get(name, capability string, mode int) (plugingetter.CompatPlug
 			}
 			// Plugin was found but it is disabled, so we should not fall back to legacy plugins
 			// but we should error out right away
-			return nil, ErrNotFound(name)
+			return nil, ErrDisabled(name)
 		}
 		if _, ok := errors.Cause(err).(ErrNotFound); !ok {
 			return nil, err