### What?
When using `throw new APIResponse("Custom error message", 500, null,
true)` the error message is being replaced with the standard "Something
went wrong" message.
### Why?
We are not checking if the 4th argument (`isPublic`) is false before
masquerading the error message.
### How?
Adds a check for `!err.isPublic` before adjusting the outgoing message.
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