OTM: Handle syntax errors at *both* stages when migrating

defaults.persistent.lua

We were pcall'ing the parsing, but not the execution...
The funky Lua syntax quirks means that it is possible to pass the former
but not the latter ;).

Fix #9700, de-facto regression since #9546

frontend/ui/data/onetime_migration.lua

@@ -456,7 +456,14 @@ if last_migration_date < 20220930 then
     if not load_defaults then
         logger.warn("loadfile:", err)
     else
-        load_defaults()
+        -- User input, there may be syntax errors, go through pcall like we used to.
+        local ok, perr = pcall(load_defaults)
+        if not ok then
+            logger.warn("Failed to execute defaults.persistent.lua:", perr)
+            -- Don't keep *anything* around, to make it more obvious that something went screwy...
+            logger.warn("/!\\ YOU WILL HAVE TO MIGRATE YOUR CUSTOM defaults.lua SETTINGS MANUALLY /!\\")
+            defaults = {}
+        end
     end
 
     for k, v in pairs(defaults) do