make `brew cask audit` return error code on fail

Per report in #3655

lib/cask/audit.rb

@@ -16,6 +16,7 @@ class Cask::Audit
     _check_no_checksums_if_latest
     _check_sourceforge_download_url_format
     _check_download(download) if download
+    return !(errors? or warnings?)
   end
 
   def summary_header

lib/cask/auditor.rb

@@ -2,12 +2,13 @@ class Cask::Auditor
   def self.audit(cask, options = {})
     audit = Cask::Audit.new(cask)
 
-    if options.fetch(:audit_download, false)
+    retval = if options.fetch(:audit_download, false)
       audit.run!(Cask::Download.new(cask))
     else
       audit.run!
     end
 
     puts audit.summary
+    retval
   end
 end

lib/cask/cli/audit.rb

@@ -4,7 +4,13 @@ class Cask::CLI::Audit
   end
 
   def self.run(*args)
-    new(args, Cask::Auditor).run
+    retval = new(args, Cask::Auditor).run
+    # retval is ternary: true/false/nil
+    if retval.nil?
+      raise CaskError.new("audit failed")
+    elsif ! retval
+      raise CaskError.new("some audits failed")
+    end
   end
 
   def initialize(args, auditor)
@@ -13,7 +19,11 @@ class Cask::CLI::Audit
   end
 
   def run
-    casks_to_audit.each { |cask| audit(cask) }
+    count = 0
+    casks_to_audit.each do |cask|
+      count += 1 if audit(cask)
+    end
+    count == 0 ? nil : count == casks_to_audit.length
   end
 
   def audit(cask)