If prevErr.isZero() is true, it will skip writing current error string.

[mritunjaykumar]

I believe there is a bug at this line https://github.com/upspin/upspin/blob/master/errors/errors.go#L265
rather than

	if e.Err != nil {
		// Indent on new line if we are cascading non-empty Upspin errors.
		if prevErr, ok := e.Err.(*Error); ok {
			if !prevErr.isZero() {
				pad(b, Separator)
				b.WriteString(e.Err.Error())
			}
		} else {
			pad(b, ": ")
			b.WriteString(e.Err.Error())
		}
	}

it should be

	if e.Err != nil {
		// Indent on new line if we are cascading non-empty Upspin errors.
		if prevErr, ok := e.Err.(*Error); ok {
			if !prevErr.isZero() {
				pad(b, Separator)
			} else {
                                 pad(b, ": ")
                         }
		} else {
			pad(b, ": ")
		}
		b.WriteString(e.Err.Error())
	}

[adg]

I believe you are correct.

[oec]

I think the original behaviour is correct, albeit confusing:

1. changing the code to the suggested or equivalent code breaks the test TestDoesNotChangePreviousError
2. if prevErr.isZero() is true, there is nothing to print.

I think it would be less confusing if the first call to WriteString would use prevErr.Error(), not e.Err.Error()