Fixes issue with error reporting when directory is not a git repo

[tommarshall]

If the current directory, or explicitly set target directory (via --repo) is not a git repository, dandelion is throwing a ruby error instead of the intended error message:

$ mkdir tmp 
$ cd tmp

$ dandelion status
/Users/tom/work/github/dandelion/lib/dandelion/cli.rb:33:in `new': Could not find repository from '/Users/tom/Desktop' (Rugged::RepositoryError)
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:33:in `repo'
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:104:in `repo_exists?'
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:111:in `validate!'
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:65:in `execute!'
  from /Users/tom/work/github/dandelion/bin/dandelion:8:in `<top (required)>'
  from /Users/tom/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bin/dandelion:23:in `load'
  from /Users/tom/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bin/dandelion:23:in `<main>'

$ dandelion --repo="/tmp" status
/Users/tom/work/github/dandelion/lib/dandelion/cli.rb:33:in `new': Could not find repository from '/Users/tom/Desktop' (Rugged::RepositoryError)
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:33:in `repo'
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:104:in `repo_exists?'
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:111:in `validate!'
  from /Users/tom/work/github/dandelion/lib/dandelion/cli.rb:65:in `execute!'
  from /Users/tom/work/github/dandelion/bin/dandelion:8:in `<top (required)>'
  from /Users/tom/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bin/dandelion:23:in `load'
  from /Users/tom/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bin/dandelion:23:in `<main>'

repo_exists? was set up to rescue ::IOError, ::Rugged::OSError, but the error being thrown is ::Rugged::RepositoryError. I've added this to the rescue.

After this change:

$ dandelion status
Not a git repository: /Users/tom/tmp
$ dandelion --repo="/tmp" status
Not a git repository: /tmp

Gem versions:

dandelion (0.4.14)
rugged (0.22.1b1)

[scttnlsn]

Thanks again!