[Resolver] Use NoSuchDependencyError

Gemfile.lock

@@ -18,7 +18,7 @@ GIT
 
 GIT
   remote: https://github.com/CocoaPods/Resolver.git
-  revision: baebceabbbeea954ca3a398794c2008aa72b9d87
+  revision: c9532593ab99e25aa1aa61c1410e72ded3c2a2f1
   branch: master
   specs:
     resolver (0.0.1)

lib/cocoapods/resolver.rb

@@ -213,8 +213,7 @@ module Pod
         end
         cached_sets[name] = set
         unless set
-          raise Informative, 'Unable to find a specification for ' \
-            "`#{dependency}`."
+          raise ::Resolver::NoSuchDependencyError.new(dependency)
         end
       end
       cached_sets[name]

spec/unit/resolver_spec.rb

@@ -122,7 +122,7 @@ module Pod
           @resolver.resolve
         end.message
         message.should.match /Unable to find a specification/
-        message.should.match /`Windows` depended upon by BlocksKit/
+        message.should.match /`Windows` depended upon by `BlocksKit`/
       end
 
       it 'does not raise if all dependencies are supported by the platform of the target definition' do