aria-ACTION-2030 Make sure error messages are visible per issue 1007

I updated the text for aria-errormessage to ensure that error messages are visible when pertinent. The text is designed to be consistent with the aria-details text regarding visibility of the extended description. 

See: https://rawgit.com/w3c/aria/issue1007/aria/aria.html#aria-errormessage

Where I added this text:
"When aria-errormessage is pertinent, authors MUST ensure the content is not hidden and is included in a container that exposes the content to the user as it is expected that the assistive technology user navigate to the content to access it.”

Rich