update: mimic doctor permissions advice
update: mimic doctor permissions advice

We might as well be consistent about it, since it's not obvious to people
what the next step is after we've pointed out the directory isn't writable.

Fixes #476.

claui/brew,jmsundar/brew,bfontaine/brew,palxex/brew,tonyg/homebrew,MikeMcQuaid/brew,zmwangx/brew,reitermarkus/brew,sjackman/homebrew,reelsense/brew,reelsense/brew,palxex/brew,vitorgalvao/brew,sjackman/homebrew,Homebrew/brew,konqui/brew,konqui/brew,muellermartin/dist,jmsundar/brew,AnastasiaSulyagina/brew,Homebrew/brew,tonyg/homebrew,palxex/brew,claui/brew,Linuxbrew/brew,zmwangx/brew,amar-laksh/brew_sudo,MikeMcQuaid/brew,reelsense/brew,muellermartin/dist,mahori/brew,zmwangx/brew,gordonmcshane/homebrew,vitorgalvao/brew,nandub/brew,AnastasiaSulyagina/brew,claui/brew,gregory-nisbet/brew,aw1621107/brew,maxim-belkin/brew,sjackman/homebrew,bfontaine/brew,JCount/brew,alyssais/brew,rwhogg/brew,hanxue/linuxbrew,JCount/brew,nandub/brew,amar-laksh/brew_sudo,toonetown/homebrew,toonetown/homebrew,reitermarkus/brew,mgrimes/brew,MikeMcQuaid/brew,reitermarkus/brew,alyssais/brew,mgrimes/brew,vitorgalvao/brew,gordonmcshane/homebrew,JCount/brew,DomT4/brew,aw1621107/brew,DomT4/brew,MikeMcQuaid/brew,ilovezfs/brew,aw1621107/brew,maxim-belkin/brew,maxim-belkin/brew,gordonmcshane/homebrew,AnastasiaSulyagina/brew,mahori/brew,Homebrew/brew,mahori/brew,rwhogg/brew,pseudocody/brew,konqui/brew,muellermartin/dist,ilovezfs/brew,EricFromCanada/brew,ilovezfs/brew,tonyg/homebrew,DomT4/brew,Homebrew/brew,toonetown/homebrew,hanxue/linuxbrew,amar-laksh/brew_sudo,reitermarkus/brew,sjackman/homebrew,alyssais/brew,gregory-nisbet/brew,DomT4/brew,mahori/brew,EricFromCanada/brew,JCount/brew,pseudocody/brew,nandub/brew,gregory-nisbet/brew,rwhogg/brew,Linuxbrew/brew,claui/brew,EricFromCanada/brew,jmsundar/brew,nandub/brew,hanxue/linuxbrew,vitorgalvao/brew,EricFromCanada/brew,pseudocody/brew,Linuxbrew/brew,mgrimes/brew,konqui/brew,bfontaine/brew,Linuxbrew/brew