fix incorrect defer exit(err) statements (#93)

When calling a function directly from a `defer` statement, like so:

```go
var err error
exit := someFuncReturningAFunctor()
defer exit(err)
```

The `exit` function is passed the `err` variable by value, which means
that the value of `err` will always be nil above.

In order to have the deferred `exit` function use the value of the `err`
variable *on exit of the function*, you need to wrap the `exit` call
inside a temporary function, like so:

```go
var err error
exit := someFuncReturningAFunctor()
defer func() {
        exit(err)
}()
```

See here for more information about this odd behaviour:

https://stackoverflow.com/questions/42703707/when-defer-func-evaluates-its-parameters

The alternative solution here was to change `exit`'s call signature to
accept a `*error` instead of an `error`, however we have code out there
in the controllers that would break if we changed `exit`'s function
signature.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.