Pipe errors through the barback asset graph

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#19491

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If a build error causes an asset not to be generated, rather than just deleting that asset from the graph barback should set it to an error state. The error should then be propagated down through all transformations of that asset, and should be exposed to the user if they call [getAsset] for a tainted asset.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Marked this as blocking dart-lang/pub#1015.