Enhance DepList error reporting capabilities for missing packages

[kugelfang]

Currently the following error is reported when a package is found that is not available:

dvandyk@phi documents $ paludis --log-level silent --install everything --dl-reinstall if-use-changed --pretend
Building target list...
Building dependency list...
Query error:
  * In program paludis --log-level silent --install everything --dl-reinstall if-use-changed --pretend:
  * When performing install action from command line:
  * When executing install task:
  * When adding PackageDepSpec 'kde-misc/vimpart':
  * No versions of 'kde-misc/vimpart' are available

Would be nice if a list of all unavailable package names would be displayed instead, as is already done for blockers. Might be viable to only show the requested behaviour for sets.

[ciaranm]

Mmm. Probably doable in the same way we do mask overrides...