Replace error messages with predefined variables for Batch

[mike-sb]

Added predifined errors for batch funcs to consume them with errors.Is.

[jackc]

I'm not necessarily opposed to this, but I do have one concern.

In general, I think of named errors as existing because the caller is going to do something based on the type of error. But as far as I can tell, all of these errors are actually caller bugs. In that case, I don't see why the caller would check for them. If they happen the caller should be changed so they never occur.

But perhaps I'm missing some use case...

[mike-sb]

I'm not necessarily opposed to this, but I do have one concern.

In general, I think of named errors as existing because the caller is going to do something based on the type of error. But as far as I can tell, all of these errors are actually caller bugs. In that case, I don't see why the caller would check for them. If they happen the caller should be changed so they never occur.

But perhaps I'm missing some use case...

I encountered a case while processing a Batch where I wanted to check if it had finished. I couldn't use errors.Is for the comparison because a new error instance was created each time. It seems to me it would be better to iterate over the array of Batch responses rather than the original input array, since we are dealing with database behavior rather than user data.