Also, improve the documentation of the fixup_bundle and fixup_bundle_item
functions to clarify that plugin type "libs" need to be copied into
the bundle *before* calling fixup_bundle.

Commit e93a4b4d changed the way that
the libs parameter to fixup_bundle is interpreted. Before the commit,
the libs were copied into the bundle first and then fixed up. After
the commit, the copy was skipped, assuming the libs were in the bundle
in the first place, and then the fixups occurred as before.

However, before the commit, it was possible to name a lib from outside
the bundle, and have it copied in and then fixed up. Its resolved
embedded name was always inside the bundle before. After, its resolved
embedded name was just the same as its resolved name, which is in its
original location, and not necessarily inside the bundle.

This manifested itself as a problem with the ParaView call to
fixup_bundle and its many plugins. Previously, ParaView had simply
passed in the list of plugin file names as they existed in the build
tree, and left the copying into the bundle up to the fixup_bundle
function. When built with CMake 2.8.3 (the first version to contain
the above named commit) the fixup_bundle call would inadventently
fixup libraries in the build tree, not libraries that were in the
bundle. Furthermore, the plugins would not be in the final bundle.

This points out the fact that the fix for the bugs made by the above
commit was a backwards-incompatible change in behavior.

This commit makes it an error to try to fixup an item that is not
already inside the bundle to make the change in behavior apparent
to folks who were depending on the prior copy-in behavior: now,
they should get an error, and hopefully, reading the new and
improved documentation, should be able to resolve it in their
projects by adding code to install or copy in such libraries prior
to calling fixup_bundle.

Whew.