Improve DeliveryException

Improve DeliveryException

The sparkpost api returns errors in the following format:
```
{
  "errors": [
    {
      "message": "Message generation rejected",
      "description": "recipient address suppressed due to customer
policy",
      "code": "1902"
    }
  ]
...
}

```

`message`, `description`, and `code` are now available on the
DeliveryException instance as: `service_message`,
`service_description`, `service_code`

The original message is preserved so it will not break any legacy
parsing written by users.

lib/sparkpost_rails/exceptions.rb

@@ -1,4 +1,17 @@
 module SparkPostRails
   class DeliveryException < StandardError
+    attr_reader :service_message, :service_description, :service_code
+
+    def initialize(message)
+      errors = [*message].first
+
+      if errors.is_a?(Hash)
+        @service_message     = errors['message']
+        @service_description = errors['description']
+        @service_code        = errors['code']
+      end
+
+      super(message)
+    end
   end
 end

spec/exceptions_spec.rb

@@ -0,0 +1,65 @@
+require 'spec_helper'
+
+describe SparkPostRails::DeliveryException do
+  subject { SparkPostRails::DeliveryException.new(error) }
+
+  describe 'string' do
+    let(:error) { 'Some delivery error' }
+
+    it 'preserves original message' do
+      begin
+        raise subject
+      rescue SparkPostRails::DeliveryException => err
+        expect(error).to eq(err.message)
+      end
+    end
+  end
+
+  describe 'array with error details' do
+    let(:error) { [{ 'message' => 'Message generation rejected', 'description' => 'recipient address suppressed due to customer policy', 'code' => '1902' }] }
+
+    it 'assigns message' do
+      expect(subject.service_message).to eq('Message generation rejected')
+    end
+
+    it 'assigns description' do
+      expect(subject.service_description).to eq('recipient address suppressed due to customer policy')
+    end
+
+    it 'assigns code' do
+      expect(subject.service_code).to eq('1902')
+    end
+
+    it 'preserves original message' do
+      begin
+        raise subject
+      rescue SparkPostRails::DeliveryException => err
+        expect(error.to_s).to eq(err.message)
+      end
+    end
+  end
+
+  describe 'array with partial details' do
+    let(:error) { [{ 'message' => 'end of world' }] }
+
+    it 'assigns message' do
+      expect(subject.service_message).to eq('end of world')
+    end
+
+    it 'assigns description' do
+      expect(subject.service_description).to be nil
+    end
+
+    it 'assigns code' do
+      expect(subject.service_code).to be nil
+    end
+
+    it 'preserves original message' do
+      begin
+        raise subject
+      rescue SparkPostRails::DeliveryException => err
+        expect(error.to_s).to eq(err.message)
+      end
+    end
+  end
+end