add custom mask upload, WIP, need more test

better handle server error

lama_cleaner/app/src/event.ts

@@ -1,6 +1,10 @@
 import mitt from 'mitt'
 
 export const EVENT_PROMPT = 'prompt'
+export const EVENT_CUSTOM_MASK = 'custom_mask'
+export interface CustomMaskEventData {
+  mask: File
+}
 
 const emitter = mitt()