Add unit error payload meta field existingIds (#385)

Author: IsaacGluck

types/common.ts

@@ -514,8 +514,11 @@ export interface UnitErrorPayload {
     code?: string
     detail?: string // to be deprecated
     details?: string
-    meta?: { supportId?: string; }
-    source?: { pointer: string; }
+    meta?: {
+        supportId?: string
+        existingIds?: string[]
+    }
+    source?: { pointer: string }
     [k: string]: unknown // allow for other keys
 }