ProtocolServer+LibProtocol: Reject unhandled URLs instead of asserting

StartDownload requests for unhandled protocols (or invalid URLs) will
now refuse to load instead of asserting. A failure code is sent back
to LibProtocol and Protocol::Client::start_download() returns nullptr.

Fixes #1604.

1 files changed, 4 insertions, 0 deletions

diff --git a/Userland/pro.cpp b/Userland/pro.cpp
index 788e42427f..51d0b4912f 100644
--- a/Userland/pro.cpp
+++ b/Userland/pro.cpp
@@ -49,6 +49,10 @@ int main(int argc, char** argv)
     auto protocol_client = Protocol::Client::construct();
 
     auto download = protocol_client->start_download(url.to_string());
+    if (!download) {
+        fprintf(stderr, "Failed to start download for '%s'\n", url_string.characters());
+        return 1;
+    }
     download->on_progress = [](u32 total_size, u32 downloaded_size) {
         dbgprintf("download progress: %u / %u\n", downloaded_size, total_size);
     };