This are handy utilities to reduce several errors into only 1 either
picking the first error or merging, if there are several, into Errorv.

It is unfortunate but an issue with Errorv was realized that even though
it satisfies the error interface, it cannot be generally worked with as
error because it (being []error) is uncomparable. Thus e.g. the following
code, if err dynamic type is Errorv, will panic:

	if err == io.EOF

It is pity Go slices are uncomparable.