[FilesBulkPinning]: Fix handling of rejection.

If the checkAvailability_ method returns a rejected promise we get the
'Uncaught (in promise) undefined' crash. This code adds gentler handling
of the rejection, by reporting it and exiting the called method.

Bug: b:289468537
Change-Id: If6ba275af82a2ba8daba3e4a2913b80e72ff826e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4680661
Commit-Queue: Bo Majewski <majewski@chromium.org>
Reviewed-by: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1169118}