Unresolved dependency warning includes source position. Fixes #528

Fixes #528.

Unresolved dependency warning is moved to `UnresolvedDependencyWarning` class including
the fail path that was added in #1467.
To display the source position, I need to access the State, so I had to move the
error processing out of `IvyActions` and add `UnresolvedDependencyWarning`, which is
aware of State.

Here's global plugin dependency sample:

```
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	::          UNRESOLVED DEPENDENCIES         ::
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	:: com.typesafe.sbt#sbt-pgp;0.100.1: not found
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 
[warn] 	Note: Some unresolved dependencies have extra attributes.  Check that these dependencies exist with the requested attributes.
[warn] 		com.typesafe.sbt:sbt-pgp:0.100.1 (sbtVersion=0.13, scalaVersion=2.10)
[warn] 
[warn] 	Note: Unresolved dependencies path:
[warn] 		com.typesafe.sbt:sbt-pgp:0.100.1 (sbtVersion=0.13, scalaVersion=2.10) (/Users/foo/dotfiles/sbt/0.13/plugins/gpg.sbt#L3-4)
[warn] 		  +- org.scala-sbt:global-plugins:0.0 (sbtVersion=0.13, scalaVersion=2.10)
```

Normal library dependency sample:

```
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	::          UNRESOLVED DEPENDENCIES         ::
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	:: foundrylogic.vpp#vpp;2.2.1: not found
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 
[warn] 	Note: Unresolved dependencies path:
[warn] 		foundrylogic.vpp:vpp:2.2.1
[warn] 		  +- org.apache.cayenne:cayenne-tools:3.0.2
[warn] 		  +- org.apache.cayenne.plugins:maven-cayenne-plugin:3.0.2 (/Users/foo/hello/build.sbt#L28)
[warn] 		  +- d:d_2.10:0.1-SNAPSHOT
```

/review @jsuereth /cc @gkossakowski