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Currently a proxy refusal to allow a WebSocket to establish a tunnel
results in an error message like:

WebSocket connection to 'ws://foo:5036/' failed: Error in connection
establishment: net::ERR_TUNNEL_CONNECTION_FAILED

It is not obvious from this message that a proxy is the cause of the
problem. Change the message to:

WebSocket connection to 'ws://foo:5036/' failed: Establishing a tunnel
via proxy server failed.

BUG=454311
TEST=net_unittests, browser_tests, layout tests

Review URL: https://codereview.chromium.org/902553002

Cr-Commit-Position: refs/heads/master@{#315257}