spec: more efficient alert dismissal across specs

only use exceptions for the exceptional cases

this shaves ~2 seconds off every selenium spec (zomg!), and
eliminate somes spurious hangs \o/

Change-Id: I7f4513620d0d13ddd5fdfa7296b2c6af555bc94b
Reviewed-on: https://gerrit.instructure.com/44383
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Michael Ziwisky <mziwisky@instructure.com>
Product-Review: Michael Ziwisky <mziwisky@instructure.com>
QA-Review: Michael Ziwisky <mziwisky@instructure.com>