uses: harden against missing formulae.

Library/Homebrew/cmd/uses.rb

@@ -14,6 +14,7 @@ module Homebrew
 
     uses = formulae.select do |f|
       used_formulae.all? do |ff|
+        begin
           if recursive
             f.recursive_dependencies.any? { |dep| dep.to_formula.name == ff.name } ||
               f.recursive_requirements.any? { |req| req.name == ff.name }
@@ -21,6 +22,10 @@ module Homebrew
             f.deps.any? { |dep| dep.to_formula.name == ff.name } ||
               f.requirements.any? { |req| req.name == ff.name }
           end
+        rescue FormulaUnavailableError => e
+          # Silently ignore this case as we don't care about things used in
+          # taps that aren't currently tapped.
+        end
       end
     end