fix accessability issues on edit external tool modal

fixes PLAT-574

test-plan:
*invalid fields be reported as invalid by the screenreader
*the first invalid field should recive focus

Change-Id: If647dd29c4faf0149a40b21cc897d8d35f9a72f7
Reviewed-on: https://gerrit.instructure.com/38599
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>