formula_installer: better handle missing conflict.

Doesn't feel like a good reason to abort installation for users if the
formula author has specified a conflict that doesn't exist. Instead,
behave more like the `TapFormulaUnavailableError` but print a message
telling people to report to the formula authors.

Closes https://github.com/Homebrew/homebrew-versions/issues/1327

Library/Homebrew/formula_installer.rb

@@ -251,10 +251,23 @@ class FormulaInstaller
       begin
         f = Formulary.factory(c.name)
       rescue TapFormulaUnavailableError
-        # If the formula name is in full-qualified name. Let's silently
+        # If the formula name is a fully-qualified name let's silently
         # ignore it as we don't care about things used in taps that aren't
         # currently tapped.
         false
+      rescue FormulaUnavailableError => e
+        # If the formula name doesn't exist any more then complain but don't
+        # stop installation from continuing.
+        opoo <<-EOS.undent
+          #{formula}: #{e.message}
+          'conflicts_with \"#{c.name}\"' should be removed from #{formula.path.basename}.
+        EOS
+        if ARGV.homebrew_developer?
+          raise
+        else
+          $stderr.puts "Please report this to the #{formula.tap} tap!"
+        end
+        false
       else
         f.linked_keg.exist? && f.opt_prefix.exist?
       end