Fix fixopt and fixopt error handling
Fix fixopt and fixopt error handling

The Keg constructor was throwing because we were returning a relative path, and the error-handler was just returning a string rather than raising a string.

Worrying because a lot of opts have not been fixed and I thought this was essential for depending on keg-only formula nowadays.

Refs Homebrew/homebrew#15077.

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