Misleading error message when an existent transformer fails to load

[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#15594

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There have been several cases when using pub build/serve where a transformer has existed but has failed to load due to networking issues (e.g. issue #729). The error message produced in these cases is "Transformer library '...' not found", which is misleading, since the library exists. We should check barback to see if the library exists, and if so print the underlying isolate load error instead.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Added Duplicate label.
Marked as being merged into #896.