crashpad_stackwalker: Check for missing minidump_stackwalk.

Error out early since the script will fail without it.

Change-Id: I2f569e91f4b29269ebc7ae9f0c1542c218c3303a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529164
Auto-Submit: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642049}
1 file changed