Add error messages for non-existent users

[sideloading]

When executing vsco-dl against a user that doesn't exist, it leads to a Traceback error. It would be nice to have a friendlier log message.

Example:

$ vsco-dl.rb userthatdoesntexist
Loading initial dataTraceback (most recent call last):
        9: from vsco-dl.rb:137:in `<main>'
        8: from vsco-dl.rb:65:in `download'
        7: from /home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:35:in `open'
        6: from /home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:736:in `open'
        5: from /home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:165:in `open_uri'
        4: from /home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:224:in `open_loop'
        3: from /home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:224:in `catch'
        2: from /home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:226:in `block in open_loop'
        1: from /home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:756:in `buffer_open'
/home/bpo5r5/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/open-uri.rb:378:in `open_http': 404 Not Found (OpenURI::HTTPError)

Same appears with the -i flag.