Allow multiple error messages for batch jobs

[m-mohr]

In #165 we added a field/response that could contain a single error message for failed batch jobs. A single error message is a good start, but multiple error messages could be even more helpful. Therefore, I'd suggest to make it an array. That also simplifies the null handling as no error is simply an empty array, similar to how the validation handles it.

[m-mohr]

As the job should be stopped after the first error, I came to the conclusion that one error should be enough... More messages can be handled with logging as proposed in #214