Provide more info if the launcher update fails (#214)

Also prevents the launcher from bricking itself (again)

---

By creating this pull request, I understand that code that is AI
generated or otherwise automatically generated may be rejected without
further discussion.
I declare that I fully understand all code I pushed into this PR, and
wrote all this code myself and own the rights to this code.

src/Startup.cpp

@@ -211,11 +211,11 @@ void CheckForUpdates(const std::string& CV) {
             error("Auto update is NOT implemented for the Linux version. Please update manually ASAP as updates contain security patches.");
 #else
             info("Downloading Launcher update " + LatestHash);
-            HTTP::Download(
+            if (HTTP::Download(
                 "https://backend.beammp.com/builds/launcher?download=true"
                 "&pk="
                     + PublicKey + "&branch=" + Branch,
-                GetBP() / (beammp_wide("new_") + GetEN()), LatestHash);
+                GetBP() / (beammp_wide("new_") + GetEN()), LatestHash)) {
                 std::error_code ec;
                 fs::remove(Back, ec);
                 if (ec == std::errc::permission_denied) {
@@ -226,6 +226,9 @@ void CheckForUpdates(const std::string& CV) {
                 }
                 fs::rename(GetBP() / (beammp_wide("new_") + GetEN()), BP);
                 URelaunch();
+            } else {
+                throw std::runtime_error("Failed to download the launcher update! Please try manually updating it, https://docs.beammp.com/FAQ/Update-launcher/");
+            }
 #endif
         } else {
             warn("Launcher update was found, but not updating because --no-update or --dev was specified.");