More cleverly split up glued error messages

[Southclaws]

Currently, if you use a library that outputs wrapped errors, you end up with duplicates.

For example:

Results in:

errorf wrapped: stdlib sentinel error: stdlib sentinel error

The stdlib sentinel error part is duplicated, which is just noise when trying to read errors.

Solution? Not sure what the best course of action is, but it seems fairly simple to just use the next error message to de-duplicate/split the current.

For example, given:

[1] errorf wrapped: stdlib sentinel error
[2] stdlib sentinel error

When processing [1], look ahead to [2] and remove [2]'s nested message from [1], so we're left with:

[1] errorf wrapped
[2] stdlib sentinel error