Provide option to disable aeson decoding error messages

When sending an invalid json payload, the backend sends this message back:
```
Error in $: When parsing the record Person of type Lib.Person the key name was not present.
```
This exposes server internals, which I find is weird to have as default behaviour.

To workaround this, someone suggested this as a solution, which is to use ``Lenient`` in my routes:
```haskell
ReqBody' '[Lenient] '[JSON] a
```
And then within my handler, I'd take in the following parameter:
```haskell
Either String a
```
While this works, it looks really ugly to have to mark every route with ``Lenient``, and every handler with ``Either String a``.
Of course, I could always write a convenience function & type to wrap over it, but is there a better solution to this?