Pub should gracefully handle transformers rejecting their configuration

[nex3]

Right now, if a transformer throws an exception due to the user passing in invalid configuration, that exception is printed with no context as an unexpected error. Pub shouldn't consider this an unexpected error, and should provide enough context for the user to determine which transformer failed and which package provided the transformer with its configuration.

This support also needs to work for the built-in dart2js transformer when issue #14664 is fixed.

[munificent]

Removed Priority-Unassigned label.
Added Priority-Medium label.

[nex3]

Set owner to @nex3.
Added Started label.

[nex3]

Fixed in r37752.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#809.