Catch exceptions in findUsableBranchers

In the version of process that comes with GHC 7.7,
readProcessWithExitCode raises an exception when
the process could not be started (for example if
the command to be run doesn't exist).
Merge pull request #1491 from rwbarton/rwbarton-get-s

"cabal get --source-repository" fails under ghc 7.7 when a brancher is missing