[Resolver] Shows an informative error message when there is no base…

[Resolver] Shows an informative error message when there is no base specification found for a `:head` dependency.

Closes https://github.com/CocoaPods/CocoaPods/issues/3230.

CHANGELOG.md

@@ -29,6 +29,11 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
   [Boris Bügling](https://github.com/neonichu)
   [#3193](https://github.com/CocoaPods/CocoaPods/issues/3193)
 
+* Shows an informative error message when there is no base specification found
+  for a `:head` dependency.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#3230](https://github.com/CocoaPods/CocoaPods/issues/3230)
+
 
 ## 0.36.0.rc.1
 

lib/cocoapods/resolver.rb

@@ -311,7 +311,7 @@ module Pod
         else
           set = create_set_from_sources(dependency)
         end
-        if dependency.head?
+        if set && dependency.head?
           set = Specification::Set::Head.new(set.specification)
         end
         cached_sets[name] = set

spec/unit/resolver_spec.rb

@@ -320,6 +320,17 @@ module Pod
         config.sandbox.head_pod?('JSONKit').should.be.true
       end
 
+      it 'raises when unable to find a base spec for a HEAD dependency' do
+        podfile = Podfile.new do
+          platform :ios, '7.0'
+          pod 'ALEKit', :head
+        end
+        resolver = Resolver.new(config.sandbox, podfile, empty_graph, SourcesManager.all)
+        Source::Aggregate.any_instance.stubs(:search).with(Dependency.new('ALEKit', :head)).returns(nil)
+        e = should.raise(Informative) { resolver.resolve.values.flatten.map(&:to_s) }
+        e.message.should.match /Unable to find a specification for `ALEKit \(HEAD\)`/
+      end
+
       it 'raises if it finds two conflicting explicit dependencies' do
         podfile = Podfile.new do
           platform :ios