Fix error messages for disjoint version errors.

Fix error messages for disjoint version errors.

BUG=https://code.google.com/p/dart/issues/detail?id=15474
R=nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=31151

[Bob Nystrom, 2013-12-13 21:17:54]

There were actually a few things wrong here:

1. On a disjoint constraint exception, we used the name of the last selected
depender and not the failed package itself in the error message.
2. That in turn conveniently avoid an issue where the backjumper would try
different versions of a package with a disjoint constraint.
3. The version solver tests for errors tested that the depender names were in
the error message, but didn't actually test that the failed package's name was
in the error.

This is all fixed now.

[nweiz, 2013-12-13 23:02:09]

lgtm

[Bob Nystrom, 2013-12-14 00:07:57]

This has the changes from the barback error patch now. PTAL.

[nweiz, 2013-12-14 00:21:54]

> I can do this if you really want me to, but it will be a pain. I'm confident
this won't happen because of the structure of the solver. The reason it's broken
into separate Solver and Traverser classes is specifically for this case. It
ensures that the entire Traverser is destroyed as soon as any backtracking
starts and all of its transitive state is discarded.

SGTM

lgtm

[Bob Nystrom, 2013-12-14 00:24:01]

Committed patchset #2 manually as r31151 (presubmit successful).