Currently, the only indication of resource packs not loading is an error in the console on startup.
I have a few ideas that some or all of which could be used:
errors.log file which generates every time a resource pack is generated if errors occur. This would just be a copy-paste of the error as it gets sent to the console.
A warning message in-game sent to either OP'd players/players with specific permission any time the server attempts to reload the resource pack and errors occur. This could also have the cause stated in the error.
A general message in game when reloading if the reload has failed (currently no output is given)
Currently, the only indication of resource packs not loading is an error in the console on startup.
I have a few ideas that some or all of which could be used: