Add extra checks to web_terminal_spec

What does this MR do and why?

Describe in detail what your merge request does and why.

Currently the test passes with a confirmed bug in place (false positive) - see #338179 (comment 670627123). The test makes an assumption that the terminal started successfully based on a component appearing, not taking into account that it might display errors within. The terminal is a Canvas element, so we can't check what text it shows, however, WebSocket connection error also appears in JS console logs so we can catch it this way.

NB the test will remain in quarantine till the bug is fixed in #331711 (closed)

Second task of https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1007

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How to set up and validate locally

To run the test on your local:

bundle exec bin/qa Test::Instance::All http://localhost:3000/ -- qa/specs/features/browser_ui/3_create/web_ide/web_terminal_spec.rb --tag quarantine

MR acceptance checklist

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