The error return from copyValue was ignored causing some XML attribute
parsing to swallow an error.

Additionally, type MyMarshalerAttrTest had no UnmarshalXMLAttr method
causing marshalTests not to be symmetrical and the test suite to fail
for test case 101.

Fixes #16158

Change-Id: Icebc505295a2c656ca4b42ba37bb0957dd7260c6
Reviewed-on: https://go-review.googlesource.com/27455Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>