Fix error handling in edge promises

* Now properly handles errors raised within deeply nested futures.
* `value!` now properly works with `rejected_future`

Fixes ruby-concurrency#659

lib/concurrent/edge/promises.rb

@@ -982,12 +982,12 @@ def value!(timeout = nil)
       # @return [Exception]
       def exception(*args)
         raise Concurrent::Error, 'it is not rejected' unless rejected?
-        reason = Array(internal_state.reason).compact
+        reason = Array(internal_state.reason).flatten.compact
         if reason.size > 1
           Concurrent::MultipleErrors.new reason
         else
           ex = reason[0].exception(*args)
-          ex.set_backtrace ex.backtrace + caller
+          ex.set_backtrace Array(ex.backtrace) + caller
           ex
         end
       end

spec/concurrent/edge/promises_spec.rb

@@ -220,9 +220,14 @@ def behaves_as_delay(delay, value)
       let(:a_future) { future { raise 'error' } }
 
       it 'raises a concurrent error' do
-        expect { zip(a_future).value! }.to raise_error(StandardError)
+        expect { zip(a_future).value! }.to raise_error(StandardError, 'error')
       end
 
+      context 'when deeply nested' do
+        it 'raises the original error' do
+          expect { zip(zip(a_future)).value! }.to raise_error(StandardError, 'error')
+        end
+      end
     end
   end
 
@@ -242,6 +247,13 @@ def behaves_as_delay(delay, value)
     end
   end
 
+  describe '.rejected_future' do
+    it 'raises the correct error when passed an unraised error' do
+      f = rejected_future(StandardError.new('boom'))
+      expect { f.value! }.to raise_error(StandardError, 'boom')
+    end
+  end
+
   describe 'Future' do
     it 'has sync and async callbacks' do
       callbacks_tester = ->(event_or_future) do