scons: Simplify the check for protoc.

scons: Simplify the check for protoc.

Collapse nesting, and constrain the try/except to only the line in that
section which might actually be expected to throw an exception when
things are working normally.

This makes the code easier to read, and won't absorb and discard
exceptions which are thrown for unexpected reasons.

Change-Id: Id3ac417ab5545bef35ed8a87d05211c8ef2f5a50
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40867
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>