Add error page for GH status validation page for bad requests

requested review from @zacharysabourin and @cguindon

@cguindon Was doing final tests, and the GH ECA status page was returning blank for errors where there was no valid installation. I've added a basic error page to point users to the helpdesk in case they believe the page should have been rendered. This shouldn't come up during regular use, as anything with a link to this endpoint will be valid, and would only occur with manual entry of the URL.

Small nitpick lol

This sentence seems a bit wordy. Not sure if there's a better way to write it

How about:

This pull-request cannot be validated because the GitHub ECA app is not installed for the specified :org/:project.

You could add links to the pull-request, the Github ECA app install page, and the org/project.

changed this line in version 2 of the diff

otherwise lgtm

Using repo outside of our install base (http://localhost:8080/git/eca/status/gh/microsoft/vscode/193369)

Using repo that doesn't exist (http://localhost:8080/git/eca/status/gh/eclipsefdn-webdev/Test-Access-Repos/85)

Attempt to revalidate closed PR (http://localhost:8080/git/eca/status/gh/eclipsefdn-webdev/Test-Access-Repo/86)

LGTM

added 1 commit

• 305e023e - Add better handling of errors in GH status page for different cases

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approved this merge request

mentioned in commit c016af9e

merged