`nerd login` doesn't recognize another nerd login process.

[dxlbnl]

When a nerd login process doesn't end (waiting on the oauth flow), and another is started. It should recognize this and give handy feedback.

~ ❯❯❯ nerd login --debug
2017/08/08 10:23:18 outputter.go:143: failed to do oauth login                                                                       
2017/08/08 10:23:18 outputter.go:147: [DEBUG] Underlying error: listen tcp 127.0.0.1:9876: bind: address already in use              
failed to spawn local server                                                                                                         
github.com/nerdalize/nerd/command.spawnServer                                                                                        
        /home/dexter/go/src/github.com/nerdalize/nerd/command/login.go:142                                                           
runtime.goexit                                                                                                                       
        /usr/local/go/src/runtime/asm_amd64.s:2197                                                                                   
failed to do oauth login                                                                                                             
github.com/nerdalize/nerd/command.(*Login).DoRun                                                                                     
        /home/dexter/go/src/github.com/nerdalize/nerd/command/login.go:65                                                            
github.com/nerdalize/nerd/command.(*Login).DoRun-fm                                                                                  
        /home/dexter/go/src/github.com/nerdalize/nerd/command/login.go:35                                                            
github.com/nerdalize/nerd/command.(*command).Run                                                                                     
        /home/dexter/go/src/github.com/nerdalize/nerd/command/command.go:121                                                         
github.com/nerdalize/nerd/vendor/github.com/mitchellh/cli.(*CLI).Run                                                                 
        /home/dexter/go/src/github.com/nerdalize/nerd/vendor/github.com/mitchellh/cli/cli.go:154                                     
main.main                                                                                                                            
        /home/dexter/go/src/github.com/nerdalize/nerd/main.go:82                                                                     
runtime.main                                                                                                                         
        /usr/local/go/src/runtime/proc.go:185                                                                                        
runtime.goexit                                                                                                                       
        /usr/local/go/src/runtime/asm_amd64.s:2197