Warning improvements when requesting bad URLs

Found while debugging some issues with add-default-catalog flow in
the launcher.

simgear/io/HTTPClient.cxx

@@ -242,8 +242,13 @@ void Client::makeRequest(const Request_ptr& r)
     if( r->isComplete() )
       return;
 
+    if (r->url().empty()) {
+        r->setFailure(EINVAL, "no URL specified on request");
+        return;
+    }
+
     if( r->url().find("://") == std::string::npos ) {
-        r->setFailure(EINVAL, "malformed URL");
+        r->setFailure(EINVAL, "malformed URL: '" + r->url() + "'");
         return;
     }
 

simgear/package/Root.cxx

@@ -778,6 +778,11 @@ bool Root::removeCatalogById(const std::string& aId)
 
 void Root::requestThumbnailData(const std::string& aUrl)
 {
+    if (aUrl.empty()) {
+        SG_LOG(SG_GENERAL, SG_DEV_WARN, "requestThumbnailData: empty URL requested");
+        return;
+    }
+
     auto it = d->thumbnailCache.find(aUrl);
     if (it == d->thumbnailCache.end()) {
         bool cachedOnDisk = d->checkPersistentCache(aUrl);