Fix reporting of build errors.

Library/Homebrew/exceptions.rb

@@ -108,7 +108,7 @@ class BuildError < Homebrew::InstallationError
     @command = cmd
     @env = ENV.to_hash
     @exit_status = es.exitstatus rescue 1
-    args = args.map{ |arg| arg.gsub " ", "\\ " }.join(" ")
+    args = args.map{ |arg| arg.to_s.gsub " ", "\\ " }.join(" ")
     super formula, "Failed executing: #{command} #{args}"
   end