bump-formula-pr: improve error messages.
bump-formula-pr: improve error messages.

- Be clearer about how URLs/tags/revisions/versions are specified in
  error messages.
- Give a better error message if `hub` isn't configured.

Homebrew/brew,reitermarkus/brew,vitorgalvao/brew,Linuxbrew/brew,reitermarkus/brew,reelsense/brew,reitermarkus/brew,vitorgalvao/brew,JCount/brew,alyssais/brew,EricFromCanada/brew,konqui/brew,mgrimes/brew,sjackman/homebrew,nandub/brew,ilovezfs/brew,konqui/brew,ilovezfs/brew,nandub/brew,konqui/brew,MikeMcQuaid/brew,reitermarkus/brew,mgrimes/brew,Linuxbrew/brew,claui/brew,mgrimes/brew,vitorgalvao/brew,pseudocody/brew,EricFromCanada/brew,claui/brew,DomT4/brew,vitorgalvao/brew,MikeMcQuaid/brew,pseudocody/brew,hanxue/linuxbrew,Homebrew/brew,aw1621107/brew,hanxue/linuxbrew,pseudocody/brew,sjackman/homebrew,sjackman/homebrew,nandub/brew,maxim-belkin/brew,mahori/brew,maxim-belkin/brew,mahori/brew,alyssais/brew,alyssais/brew,DomT4/brew,mahori/brew,nandub/brew,claui/brew,ilovezfs/brew,aw1621107/brew,MikeMcQuaid/brew,JCount/brew,hanxue/linuxbrew,EricFromCanada/brew,konqui/brew,claui/brew,DomT4/brew,reelsense/brew,MikeMcQuaid/brew,Homebrew/brew,DomT4/brew,EricFromCanada/brew,Homebrew/brew,reelsense/brew,Linuxbrew/brew,aw1621107/brew,JCount/brew,sjackman/homebrew,mahori/brew,JCount/brew,Linuxbrew/brew,maxim-belkin/brew