Service Worker: Improve error messages from register().
This augments the result from RegisterJob with an optional string explaning
the status. Previously we could only convey rather generic errors.
Sample improvements:
Before: "Operation failed by network issue"
After: "A bad HTTP response code (404) was received when fetching the script."
Before: "Operation failed by security issue"
After: "The script has an unsupported MIME type ('text/plain')."
BUG=
436928
TBR=mek@ for trivial change to geofencing unittest
Committed:
https://crrev.com/3008af5bdb59ef3faca7444f3a121026662667ac
Cr-Commit-Position: refs/heads/master@{#312114}