Don't error out when a package is already installed

Fixes Homebrew/homebrew#17010.

Library/Homebrew/cmd/install.rb

@@ -91,6 +91,8 @@ module Homebrew extend self
   rescue FormulaInstallationAlreadyAttemptedError
     # We already attempted to install f as part of the dependency tree of
     # another formula. In that case, don't generate an error, just move on.
+  rescue FormulaAlreadyInstalledError => e
+    opoo e.message
   rescue CannotInstallFormulaError => e
     ofail e.message
   end

Library/Homebrew/exceptions.rb

@@ -71,6 +71,9 @@ end
 class CannotInstallFormulaError < RuntimeError
 end
 
+class FormulaAlreadyInstalledError < RuntimeError
+end
+
 class FormulaInstallationAlreadyAttemptedError < Homebrew::InstallationError
   def message
     "Formula installation already attempted: #{formula}"

Library/Homebrew/formula_installer.rb

@@ -34,7 +34,7 @@ class FormulaInstaller
     if f.installed?
       msg = "#{f}-#{f.installed_version} already installed"
       msg << ", it's just not linked" if not f.linked_keg.symlink? and not f.keg_only?
-      raise CannotInstallFormulaError, msg
+      raise FormulaAlreadyInstalledError, msg
     end
 
     # Building head-only without --HEAD is an error