brew.rb: don't exit when failing to set devcmdrun.
brew.rb: don't exit when failing to set devcmdrun.

It's not necessary to make this a hard failure so don't (and this makes
it more consistent with `brew.sh`).

Fixes #1462.

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