Adds better error for missing provider. Closes #536

lib/omniauth/builder.rb

@@ -19,7 +19,11 @@ module OmniAuth
       if klass.is_a?(Class)
         middleware = klass
       else
-        middleware = OmniAuth::Strategies.const_get("#{OmniAuth::Utils.camelize(klass.to_s)}")
+        begin
+          middleware = OmniAuth::Strategies.const_get("#{OmniAuth::Utils.camelize(klass.to_s)}")
+        rescue NameError
+          raise LoadError, "Could not find matching strategy for #{klass.inspect}. You may need to install an additional gem (such as omniauth-#{klass})."
+        end
       end
 
       use middleware, *args, &block

spec/omniauth/builder_spec.rb

@@ -16,5 +16,13 @@ describe OmniAuth::Builder do
         provider ::ExampleClass
       end }.should_not raise_error
     end
+
+    it "should raise a helpful LoadError messgae if it can't find the class" do
+      expect {
+        OmniAuth::Builder.new(nil) do
+          provider :lorax
+        end
+      }.to raise_error(LoadError, "Could not find matching strategy for :lorax. You may need to install an additional gem (such as omniauth-lorax).")
+    end
   end
 end