Improve 'window.{atob,btoa}' exception messages.

Rather than throwing generic messages, we should do a better job
explaining to the developer why her code failed to executed as she
expected it to. This patch adds detail to the various exceptions
that 'window.atob' and 'window.btoa' can throw.

BUG=270033

Review URL: https://chromiumcodereview.appspot.com/22645010

git-svn-id: svn://svn.chromium.org/blink/trunk@155865 bbb929c8-8fbe-4397-9dbb-9b2b20218538