Merge try::file fixes into master

those commits fix #3201 (closed) ("webapp: rake assets:precompile: can't convert nil into Hash"):

commit 3a12f829316b7ccaf353158ea58d27d6fd400065
Author: elijah 
Date:   Fri Jul 19 10:55:47 2013

    try::file - absolute exec paths.

commit e52f2191b616f77ffaf94152f3241ea017c296e3
Author: elijah 
Date:   Mon Jul 15 16:16:35 2013

    an entirely different implementation of try::file, using all execs. the built in file resource of puppet can't be used for what we want, because if you specify $source, it always bombs out if it doesn't exist, regardless of dependencies.

we need to merge them into master

(from redmine: created on 2013-07-19, closed on 2013-08-13, relates #3201 (closed), relates #3228 (closed), blocks #3238 (closed))