Better error output when background page fails to load.
Better error output when background page fails to load.

BUG=37082
TEST=Try loading a test extension with a background_page entry in your 
manifest.json for a file that doesn't exist. You should see the name from your
manifest in the error output.

Review URL: http://codereview.chromium.org/660287

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@40328 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser