[WebIDE] Handle VSCode workbench creation errors gracefully

MRs:

• feat: Handle VSCode workbench creation errors g... (!530 - merged)
• [WebIDE] Handle VSCode workbench creation error... (gitlab!195611 - merged)

Problem to solve

When the Web IDE VSCode Workbench fails to load, the Gitlab application displays a blank screen.

Proposed solution

Handle VSCode workbench load failures gracefully by displaying a meaningful message to the user.

Acceptance criteria

• If the VSCode Workbench iframe fails to obtain the authentication port channel, handle this error gracefully and communicate it to the user.
• Ensure that the GitLab application provides a handleError function that displays a UI message indicating that the Web IDE could not be started.

References

The following discussion from !397 (merged) should be addressed:

• @cindy-halim started a discussion:

  question: if the requested port channel does not exist a timeout error occurs when waiting for message. should we handle for this case more gracefully?