Font-Awesome icons not showing -- missing class=fa?
It sounds like you've identified a bug &mdash; and found a fix. Excellent! 

I can confirm this bug by observing the fatal error message displaying a funky character before "Sorry" as shown below. 



Here are some next steps that I'd recommend for you here:


Report the bug in CiviCRM's issue-tracker, Jira. If you're new to reporting issues, read over these issue-reporting guidelines in our Developer Guide.
After reporting the bug, you can choose to stop there and wait for someone else to fix it. But, given that this issue has a relatively minor impact, developers will be unlikely to volunteer their time on it unless you offer funding to the Core Team or to an independent expert.
Since you've already dug into the code yourself, I'd encourage you to work on contributing a fix. Here are some tips: 


Use git blame (possibly via GitHub's web-based git blame) on that line of code to see if there are recent commits which are relevant to the issue. Ah ha! In this case we can see that  someone recently made a change to "use class crm-i instead of fa".
It would be prudent to look deeper into that commit to try and understand it. I haven't gone too deep, but I did notice that the commit also changed the warning at the top of the "Option Groups" page (/civicrm/admin/options?reset=1), and that message seems to be displaying okay right now: 


From the above, I'd recommend considering whether your fix should be limited only to the fatal error page (which might be behaving differently than other pages due to different CSS being included in it)
If you're stuck or want input from other developers, hop on Mattermost.



All-in-all, this answer is just intended to be a push in the right direction towards using other tools (i.e. Jira, GitHub, and Mattermost instead of StackExchange) for reporting (and hopefully fixing) this bug. In some ways you're lucky to have found a bug which will (probably) be super easy to fix! Fixing this bug would be a great stepping stone towards improving CiviCRM in bigger ways too!