[NaCl SDK] Print nice error if host build can't find gcc or cl.exe

Previously, the build would fail with when at compile time.

BUG=none
R=noelallen@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/12220085

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@181725 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed