Element-R: detect "withheld key" UTD errors, and mark them as such (#4302)

Partial fix to element-hq/element-web#27653

src/rust-crypto/rust-crypto.ts

@@ -1787,6 +1787,17 @@ class EventDecryptor {
             }
         }
 
+        // If we got a withheld code, expose that.
+        if (err.maybe_withheld) {
+            // Unfortunately the Rust SDK API doesn't let us distinguish between different withheld cases, other than
+            // by string-matching.
+            const failureCode =
+                err.maybe_withheld === "The sender has disabled encrypting to unverified devices."
+                    ? DecryptionFailureCode.MEGOLM_KEY_WITHHELD_FOR_UNVERIFIED_DEVICE
+                    : DecryptionFailureCode.MEGOLM_KEY_WITHHELD;
+            throw new DecryptionError(failureCode, err.maybe_withheld, errorDetails);
+        }
+
         switch (err.code) {
             case RustSdkCryptoJs.DecryptionErrorCode.MissingRoomKey:
                 throw new DecryptionError(