Throw an error if HTTP file transfers are cleanly closed early

It seems that some HTTP servers can silently close the connection before
the file has finished transferring. Since we count the transferred bytes
anyway, throw an exception if we didn't get the number of bytes the
server previously told us to expect.

launcher/src/main/java/com/skcraft/launcher/util/HttpRequest.java

@@ -328,6 +328,11 @@ public class HttpRequest implements Closeable, ProgressObservable {
                 readBytes += len;
                 checkInterrupted();
             }
+
+            if (contentLength >= 0 && contentLength != readBytes) {
+                throw new IOException(String.format("Connection closed with %d bytes transferred, expected %d",
+                        readBytes, contentLength));
+            }
         } finally {
             close();
         }