Add audit warning if previous Cask version cannot be read.

Library/Homebrew/cask/lib/hbc/audit.rb

@@ -70,12 +70,16 @@ module Hbc
       previous_cask_contents = Git.last_revision_of_file(tap.path, @cask.sourcefile_path, before_commit: commit_range)
       return if previous_cask_contents.empty?
 
+      begin
         previous_cask = CaskLoader.load_from_string(previous_cask_contents)
 
         return unless previous_cask.version == cask.version
         return if previous_cask.sha256 == cask.sha256
 
         add_error "only sha256 changed (see: https://github.com/caskroom/homebrew-cask/blob/master/doc/cask_language_reference/stanzas/sha256.md)"
+      rescue CaskError => e
+        add_warning "Skipped version and checksum comparison. Reading previous version failed: #{e}"
+      end
     end
 
     def check_version