Handle comments made on commits that no longer exist

[Tyriar]

PR xtermjs/xterm.js#1731
Windows 10, ext v0.2.1

[RMacfarlane]

Looks like the commit the comment was made on doesn't exist anymore due to a force push. The web UI displays a nice message when trying to view changes for the conversation: https://github.com/xtermjs/xterm.js/pull/1731/files/43e9911bd8a34e115963b8b0bf84aa54789a9e41

We should display a better error message