Push: Add applicationServerKey to sender ID missing error messages

The PUSH_REGISTRATION_STATUS_NO_SENDER_ID and
PUSH_REGISTRATION_STATUS_MANIFEST_EMPTY_OR_MISSING error messages
are misleading now that a key or sender ID can be provided via
applicationServerKey instead of the gcm_sender_id manifest
property. Updated error messages accordingly.

BUG=463113,583753

Review-Url: https://codereview.chromium.org/2152823003
Cr-Commit-Position: refs/heads/master@{#405788}
3 files changed