preserve value for storage errors on add and put

src/matrix/storage/common.js

@@ -13,7 +13,7 @@ export const STORE_MAP = Object.freeze(STORE_NAMES.reduce((nameMap, name) => {
 }, {}));
 
 export class StorageError extends Error {
-    constructor(message, cause) {
+    constructor(message, cause, value) {
         let fullMessage = message;
         if (cause) {
             fullMessage += ": ";
@@ -26,5 +26,7 @@ export class StorageError extends Error {
         if (cause) {
             this.errcode = cause.name;
         }
+        this.cause = cause;
+        this.value = value;
     }
 }

src/matrix/storage/idb/store.js

@@ -93,12 +93,22 @@ export default class Store extends QueryTarget {
         return new QueryTarget(new QueryTargetWrapper(this._idbStore.index(indexName)));
     }
 
-    put(value) {
-        return reqAsPromise(this._idbStore.put(value));
+    async put(value) {
+        try {
+            return await reqAsPromise(this._idbStore.put(value));
+        } catch(err) {
+            const originalErr = err.cause;
+            throw new StorageError(`put on ${this._idbStore.name} failed`, originalErr, value);
+        }
     }
 
-    add(value) {
-        return reqAsPromise(this._idbStore.add(value));
+    async add(value) {
+        try {
+            return await reqAsPromise(this._idbStore.add(value));
+        } catch(err) {
+            const originalErr = err.cause;
+            throw new StorageError(`add on ${this._idbStore.name} failed`, originalErr, value);
+        }
     }
 
     delete(keyOrKeyRange) {