Better override error message.
Merge pull request #58 from djpate/master

Better override error message.
Revert "Merge pull request #58 from djpate/master"

This reverts commit 9487d458ccf1bc49970f695e9814b2bac0140dba, reversing
changes made to e8ab4bb3e56f8fcd3e03f485b6715fc11e8c7c29.
Revert "Merge pull request #57 from egeste/browserify"

This reverts commit e8ab4bb3e56f8fcd3e03f485b6715fc11e8c7c29, reversing
changes made to 4f78cae5bba2acbfecf0e791d5ef60e290ed261f.
handleMixins should iterate forward when invoking mixin.mixconfig