"pub global run" errors out for a mismatched SDK.

Previously, "pub global run" would happily try to run an executable with
an SDK constraint that doesn't match the current SDK, even though it
probably wouldn't work (and could fail in confusing ways). Now it errors
out with a user-friendly message instead.

Closes #1353

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1427523005 .