fix: prevent confusing error messages when installed: false (#508)

This removes `release: "your_release_name" not found` errors seen for releases with `installed: false` when running `helmfile sync` and `helmfile apply`.

The problem was that helmfile had been running `helm status` to detect releases to be deleted. helmfile now use `helm list ^YOUR_RELEASE_NAME$` to detect if the release is currently installed or not, which emits no error-like logs on against uninstalled releases.

Fixes #507

Fixes #507

helmexec/helmexec.go

@@ -17,6 +17,6 @@ type Interface interface {
 	ReleaseStatus(name string) error
 	DeleteRelease(name string, flags ...string) error
 	TestRelease(name string, flags ...string) error
-
+	List(filter string) (string, error)
 	DecryptSecret(name string) (string, error)
 }