Fixes "TypeError" for all http 404 responses

Prior to this PR, any `404` response from mailgun would cause a
`TypeError: exception class/object expected` to be thrown from the
mailgun gem.

What was happening is that the `#submit` method in `Mailgun::Base` would
translate any `ClientError` to a `Mailgun::Error` (which does not
inherit from `Exception`, and thus is not re-raisable) and then either
re-raise that `Mailgun::Error`'s `#handle` or the error itself, based on
whether `handle` returned an instance of `Mailgun::ErrorBase` or not.

The slightly confusing thing here is that re-raising the
`Mailgun::Error` class would always cause a `TypeError: exception
class/object expected` exception, so that code path is never desired.

There is either a bug in the inheritance of `Mailgun::Error`, and it
**should** inherit from `Exception` (or, preferably, `StandardError`),
which would allow it to be thrown directly, or we should remove the code
path that throws this unthrowable class.

Since `Mailgun::NotFound` was the only error class that seemed to take
advantage of this option, and that in the case of a 404 response a
`Mailgun::NotFound` exception seems a better error to throw, I propose
in this PR to remove the option alltogether, and always throw the result
of `Mailgun::Error#handle`.

lib/mailgun/base.rb

@@ -102,11 +102,7 @@ def self.submit(method, url, parameters={})
         :code => error_code || nil,
         :message => error_message || nil
       )
-      if error.handle.kind_of? Mailgun::ErrorBase
-        raise error.handle
-      else
-        raise error
-      end
+      raise error.handle
     end
   end
 

lib/mailgun/mailgun_error.rb

@@ -38,9 +38,6 @@ def handle
   end
 
   class NotFound < ErrorBase
-    def handle
-      return nil
-    end
   end
 
   class BadRequest < ErrorBase

spec/base_spec.rb

@@ -67,6 +67,37 @@
 
         Mailgun.submit :test_method, '/', :arg1=>"val1"
       end
+
+      context "when the client raises a 400 http error" do
+        it "should re-raise a bad request mailgun error" do
+          client_error = Mailgun::ClientError.new
+          client_error.http_code = 400
+          client_error.http_body = { "message" => "Expression is missing" }.to_json
+          expect(Mailgun::Client).to receive(:new)
+            .with('/')
+            .and_return(client_double)
+          expect(client_double).to receive(:test_method)
+            .with({:arg1=>"val1"})
+            .and_raise(client_error)
+
+          expect { Mailgun.submit :test_method, '/', :arg1=>"val1" }.to raise_exception Mailgun::BadRequest
+        end
+      end
+      context "when the client raises a 404 http error" do
+        it "should re-raise a not found mailgun error" do
+          client_error = Mailgun::ClientError.new
+          client_error.http_code = 404
+          client_error.http_body = { "message" => "Not Found" }.to_json
+          expect(Mailgun::Client).to receive(:new)
+            .with('/')
+            .and_return(client_double)
+          expect(client_double).to receive(:test_method)
+            .with({:arg1=>"val1"})
+            .and_raise(client_error)
+
+          expect { Mailgun.submit :test_method, '/', :arg1=>"val1" }.to raise_exception Mailgun::NotFound
+        end
+      end
     end
   end