tap: more thoroughly validate tap names
tap: more thoroughly validate tap names

`Tap.fetch` and commands building on top of it accepted tap names like
`homebrew/homebrew/bogus` causing some misbehavior. Ensure neither
`user` nor `repo` include slashes and print a more helpful error message
that additionally includes the problematic tap name.

Closes #585.

Closes #591.

Signed-off-by: Martin Afanasjew <54669547a225ff20cba8b75a4adca540eef25858@afanasjew.de>

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