If there is an uri then provide better error message while combining javascripts into one

- j = []
- j << 'https://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.js'
- j << 'vendor/rails'
- j << 'application'
= javascript_include_tag j, :cache => 'all'

Above code will work fine in development mode and it will fail only in production mode.

It will work fine in development mode because if it is uri then absolute path is not calculated. However it fails in production mode with following error message which does not provide any clue in terms of what went wrong.

"can't convert nil into String"

Attached is a patch with test.

After the patch the error message would be:

Asset file https://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.js is uri and cannot be merged into single file