read_profile(): don't fail silently
If a profile file can't be read, print a warning so that the user is aware of the problem.
Backporting to 2.11..2.13 might make sense especially for aa-mergeprof users. Without this warning, typos in filenames given as parameters will result in silently ignoring that file. (In master, aa-mergeprof will fail hard if a given file doesn't exist - but with a not-so-useful error message. Guess why I'm adding the warning...)
That said - when reviewing, please also indicate if / to which versions I should backport.