Scons: Make the "--mode" option stricter
Scons: Make the "--mode" option stricter

Give an error when an unknown environment is listed in "--mode" rather
than a warning.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1471
TEST="./scons --mode=foo"

Review URL: http://codereview.chromium.org/6592012

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@4404 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client