More descriptive error for unparseable DateTime

Alters the error raised when a DateTime cannot be parsed to include the
unparseable value in order to help track down the source of the problem.

lib/icalendar/values/date_time.rb

@@ -12,7 +12,14 @@ class DateTime < Value
       def initialize(value, params = {})
         if value.is_a? String
           params['tzid'] = 'UTC' if value.end_with? 'Z'
-          super ::DateTime.strptime(value, FORMAT), params
+
+          begin
+            parsed_date = ::DateTime.strptime(value, FORMAT)
+          rescue ArgumentError => e
+            raise ArgumentError.new("Failed to parse \"#{value}\" - #{e.message}")
+          end
+
+          super parsed_date, params
         elsif value.respond_to? :to_datetime
           super value.to_datetime, params
         else

spec/values/date_time_spec.rb

@@ -76,5 +76,13 @@
         expect(subject.to_ical described_class).to eq ":#{value}"
       end
     end
+
+    context 'unparseable time' do
+      let(:value) { 'unparseable_time' }
+
+      it 'raises an error including the unparseable time' do
+        expect { subject }.to raise_error(ArgumentError, %r{Failed to parse \"#{value}\"})
+      end
+    end
   end
-end
+end