webapk: Don't swallow onbind exceptions

Browser side can't deal with returning null from onbind. Plus any
exception there is already catastrophic, there is no way to recover from
that anyway.

BUG=691846

Review-Url: https://codereview.chromium.org/2699443003
Cr-Commit-Position: refs/heads/master@{#450580}
1 file changed