FIX: Show a useful message when starting a plugin fails without a backtrace

lib/plugin_initialization_guard.rb

@@ -6,6 +6,7 @@ def plugin_initialization_guard(&block)
   rescue => error
     plugins_directory = Rails.root + 'plugins'
 
+    if error.backtrace
       plugin_path = error.backtrace_locations.lazy.map do |location|
         Pathname.new(location.absolute_path)
           .ascend
@@ -33,6 +34,16 @@ def plugin_initialization_guard(&block)
 
         Please try removing this plugin and rebuilding again!
       MESSAGE
+    else
+      STDERR.puts <<~MESSAGE
+        ** PLUGIN FAILURE **
+
+        You are unable to build Discourse due to this error during plugin
+        initialization:
+
+        #{error}
+      MESSAGE
+    end
     exit 1
   end
 end