Remove use of rescue nil.

* `rescue nil` is a really bad pattern to use in our code base.
  We should rescue errors that we expect the code to throw and
  not rescue everything because we're unsure of what errors the
  code would throw. This would reduce the amount of pain we face
  when debugging why something isn't working as expexted. I've
  been bitten countless of times by errors being swallowed as a
  result during debugging sessions.

lib/stylesheet/manager.rb

@@ -110,7 +110,11 @@ class Stylesheet::Manager
       if File.exists?(stylesheet_fullpath)
         unless StylesheetCache.where(target: qualified_target, digest: digest).exists?
           begin
-            source_map = File.read(source_map_fullpath) rescue nil
+            source_map = begin
+              File.read(source_map_fullpath)
+            rescue Errno::ENOENT
+            end
+
             StylesheetCache.add(qualified_target, digest, File.read(stylesheet_fullpath), source_map)
           rescue => e
             Rails.logger.warn "Completely unexpected error adding contents of '#{stylesheet_fullpath}' to cache #{e}"