1. Open preferences.txt from C:\Users\AppData\Local\Arduino15 ( this path may vary depending on your
OS) and remove value for "boardsmanager.additional.urls"
2. Delete all .tmp files from this folder (C:\Users\AppData\Local\Arduino15)
3. Copy & paste URL in File->preferences
4. Goto Board-> Manager board... now you dont see any error and ESP8266 should be there in the list