Check existence of target keg instead of hiding exceptions

Library/Homebrew/cmd/upgrade.rb

@@ -54,7 +54,7 @@ module Homebrew extend self
   end
 
   def upgrade_formula f
-    outdated_keg = Keg.new(f.linked_keg.realpath) rescue nil
+    outdated_keg = Keg.new(f.linked_keg.realpath) if f.linked_keg.directory?
 
     fi = FormulaInstaller.new(f)
     fi.options             = Tab.for_formula(f).used_options

Library/Homebrew/formula_installer.rb

@@ -351,7 +351,7 @@ class FormulaInstaller
   def install_dependency(dep, inherited_options)
     df = dep.to_formula
 
-    outdated_keg = Keg.new(df.linked_keg.realpath) rescue nil
+    outdated_keg = Keg.new(df.linked_keg.realpath) if df.linked_keg.directory?
 
     fi = DependencyInstaller.new(df)
     fi.options           |= Tab.for_formula(df).used_options