Change default message for InvalidCharacterError.

Currently we report:

An invalid or illegal character was specified, such as in an XML name.

Firefox reports:

String contains an invalid character

The spec uses:

The string contains invalid characters.

BUG=261249
R=abarth@chromium.org,arv@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22195002

git-svn-id: svn://svn.chromium.org/blink/trunk@155662 bbb929c8-8fbe-4397-9dbb-9b2b20218538