Initial port of Array.p.join came before Torque had exception
handling.  This small cleanup also simplifies a future CL
porting TypedArray.p.join.

Bug: v8:7624
Change-Id: I74f3880ee6e87917bc87e41d94be1a83b039384b
Reviewed-on: https://chromium-review.googlesource.com/c/1347514Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Cr-Commit-Position: refs/heads/master@{#57712}