Pub should gracefully handle transformers rejecting their configuration

[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#16008

---

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 #694 is fixed.

[DartBot]

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

---

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

[DartBot]

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

---

Set owner to @nex3.
Added Started label.

[DartBot]

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

---

Fixed in dart-lang/sdk@d173793.

---

Added Fixed label.