Throw a nice error when attempting to set the account outside of a git project

[searls]

I accidentally tried setting the account from the wrong directory, and while it tried changing the git remote, it blew up instead of giving a decent error message. Would it be better to print an error message here that you must be in a git project with a heroku remote?

$ heroku accounts:set personal
 !    Heroku client internal error.
 !    Search for help at: https://help.heroku.com
 !    Or report a bug at: https://github.com/heroku/heroku/issues/new

    Error:       undefined method `each' for nil:NilClass (NoMethodError)
    Backtrace:   /Users/justin/.heroku/plugins/heroku-accounts/lib/accounts/heroku/command/accounts.rb:112:in `set'
                 /Users/justin/.heroku/client/lib/heroku/command.rb:217:in `run'
                 /Users/justin/.heroku/client/lib/heroku/cli.rb:40:in `start'
                 /usr/local/heroku/bin/heroku:24:in `<main>'

    Command:     heroku accounts:set testdouble
    Plugins:     heroku-accounts
    Version:     heroku-toolbelt/3.17.1 (x86_64-darwin10.8.0) ruby/1.9.3

[ddollar]

Yep, that sounds like it'd be a nice change. If you're interested in putting together a pull request I'd merge it.

[searls]

Submitted via the web editor, like a boss.