Commit 79e328c5 (xerr += First, Merge) was not right saying that

	if err == io.EOF

will panic if err has dynamic type Errorv. It will not because
interfaces with different dynamic types are always not equal.
However Errorv == Errorv will indeed panic.

Document and test that it is safe to compare error vectors to other
errors, e.g. to io.EOF, and that Errorv == Errorv panics.