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In addition to a developer-readable string for inline install failures, include
a stable error code. This isn't perfect, since a) the function prototype of
function(error, errorCode) isn't great, and b) we kind of lose track of what
went wrong after a certain point in the install process. Unfortunately, to fix
a) requires either a breaking change or a revamp to our whole error reporting,
and fixing b) requires stable error values at every point in our install
process (which we don't have, and won't anytime soon).

BUG=331887
TBR=benwells@chromium.org (app_list)
TBR=yoshiki@chromium.org (file_manager)

TBRs for trivial, no-functionality changes.

Review URL: https://codereview.chromium.org/384823002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285029 0039d316-1c4b-4281-b951-d872f2087c98