Don't send messages with restoring attachments.

SignalServiceKit/src/Messages/OWSMessageSender.m

@@ -184,7 +184,11 @@ void AssertIsOnSendingQueue()
     if (self.message.hasAttachments) {
         [self.dbConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
             for (TSAttachment *attachment in [self.message attachmentsWithTransaction:transaction]) {
-                OWSAssertDebug([attachment isKindOfClass:[TSAttachmentStream class]]);
+                if (![attachment isKindOfClass:[TSAttachmentStream class]]) {
+                    return OWSErrorWithCodeDescription(
+                        OWSErrorCodeMessageHasInvalidAttachments, @"Message with missing attachments cannot be sent.");
+                }
+
                 TSAttachmentStream *attachmentStream = (TSAttachmentStream *)attachment;
                 OWSAssertDebug(attachmentStream);
                 OWSAssertDebug(attachmentStream.serverId);

SignalServiceKit/src/Util/OWSError.h

@@ -51,6 +51,7 @@ typedef NS_ENUM(NSInteger, OWSErrorCode) {
     OWSErrorCodeProfileUpdateFailed = 777424,
     OWSErrorCodeAvatarWriteFailed = 777425,
     OWSErrorCodeAvatarUploadFailed = 777426,
+    OWSErrorCodeMessageHasInvalidAttachments,
 };
 
 extern NSString *const OWSErrorRecipientIdentifierKey;