fixes issue where dagger hands when input directory does not exist

Signed-off-by: Richard Jones <richard@dagger.io>

tests/plan/inputs/directories/not_exists.cue

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"alpha.dagger.io/europa/dagger/engine"
+)
+
+engine.#Plan & {
+	// should fail because path does not exist locally
+	inputs: directories: test: path: "./fasdfsdfs"
+	actions: verify: engine.#ReadFile & {
+		input: inputs.directories.test.contents
+		path:  "test.txt"
+	} & {
+		contents: "local directory"
+	}
+}