Catch exception thrown when cancelling fetch in grpc-web client

[jrhee17]

grpc-web-client.umd.js:1 Uncaught (in promise) DOMException: The user aborted a request.

I'm seeing this log in the browser when cancelling a grpc-web request. Since reader.cancel() returns a promise, I am guessing the cancellation is scheduled, and subsequently logged.

Changes

Add a catch statement when aborting a fetch response reader.

Verification

Verified from my local, I am assuming regressions will be checked via existing unit tests.

grpc-web-client.umd.js:1 onHeaders.gRPCStatus 16
grpc-web-client.umd.js:1 onHeaders.code 16
grpc-web-client.umd.js:1 onHeaders.gRPCMessage [...]
grpc-web-client.umd.js:1 rawOnHeaders e {headersMap: {…}}
pollEpic.ts:39 e {headersMap: {…}}
grpc-web-client.umd.js:1 rawOnError 16 user doesn't exist
grpc-web-client.umd.js:1 request.abort aborting request
grpc-web-client.umd.js:1 Fetch.abort.cancel before reader
grpc-web-client.umd.js:1 Fetch.pump.cancel at first pump
grpc-web-client.umd.js:1 Fetch.pump.cancel caught DOMException: The user aborted a request.

[improbable-prow-robot]

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