Introduction

Even though grum wrote in his comment on MC-86949:

Using internals gives you internal errors

(Not sure if this applies here)
There are enough situations in which plain exceptions are printed in the log without a message like "Couldn't process command".

The bug

Using commands which do not run successfully because of an exception while executing them only prints the following in the log.

Couldn't process command: '[COMMAND USED]'

This makes it pretty difficult to find the reason why this happened. Instead the exception should be printed as well.

How to reproduce

1. Use the reproduction steps of MC-116927 while having the world open
2. Use the /reload command

You cannot tell what why the command could not be executed. (In this case the advancement file had an invalid content)