nettle: update 2.7.1 bottle.
giflib: update 4.1.6 bottle.
gnutls: update 3.2.12.1 bottle.
yajl: update 2.0.4 bottle.
Rework make_relative_symlink error handling and move it into keg
Raise useful errors from make_relative_symlink
Only unlink after a LinkError
Raise AlreadyLinkedError when a keg is already linked
Adjust link command for updated error handling
Adjust installer for updated link error handling