Put Tinter loop body in a try/catch

So whatever other random ways this process fails in don't cause it
to take out the whole app.

src/Tinter.js

@@ -298,6 +298,7 @@ class Tinter {
 
         for (let i = 0; i < document.styleSheets.length; i++) {
             const ss = document.styleSheets[i];
+            try {
                 if (!ss) continue; // well done safari >:(
                 // Chromium apparently sometimes returns null here; unsure why.
                 // see $14534907369972FRXBx:matrix.org in HQ
@@ -351,6 +352,13 @@ class Tinter {
                         }
                     }
                 }
+            } catch (e) {
+                // Catch any random exceptions that happen here: all sorts of things can go
+                // wrong with this (nulls, SecurityErrors) and mostly it's for other
+                // stylesheets that we don't want to proces anyway. We should not propagate an
+                // exception out since this will cause the app to fail to start.
+                console.log("Failed to calculate CSS fixups for a stylesheet: " + ss.href, e);
+            }
         }
         if (DEBUG) {
             console.log("calcCssFixups end (" +