uses: restore formula-level exception handler

This is a temporary measure until:

- #1862 is merged
- I can a test on `brew uses` against every formula in the official
  taps to verify that exceptions are no longer raised.

Library/Homebrew/cmd/uses.rb

@@ -47,6 +47,7 @@ module Homebrew
 
     uses = formulae.select do |f|
       used_formulae.all? do |ff|
+        begin
           if recursive
             deps = f.recursive_dependencies do |dependent, dep|
               if dep.recommended?
@@ -91,6 +92,10 @@ module Homebrew
           reqs.any? do |req|
             req.name == ff.name || [ff.name, ff.full_name].include?(req.default_formula)
           end
+        rescue FormulaUnavailableError
+          # Silently ignore this case as we don't care about things used in
+          # taps that aren't currently tapped.
+        end
       end
     end