FIX: save email rejection error class names for incoming email logs (#16095)

lib/email/receiver.rb

@@ -83,9 +83,7 @@ module Email
 
           post
         rescue Exception => e
-          error = e.to_s
-          error = e.class.name if error.blank?
-          @incoming_email.update_columns(error: error) if @incoming_email
+          @incoming_email.update_columns(error: e.class.name) if @incoming_email
           delete_staged_users
           raise
         end

spec/lib/email/processor_spec.rb

@@ -120,7 +120,7 @@ describe Email::Processor do
         expect(errors.first).to include("boom")
 
         incoming_email = IncomingEmail.last
-        expect(incoming_email.error).to eq("boom")
+        expect(incoming_email.error).to eq("RuntimeError")
         expect(incoming_email.rejection_message).to be_present
 
         expect(EmailLog.last.email_type).to eq("email_reject_unrecognized_error")

spec/lib/email/receiver_spec.rb

@@ -98,6 +98,7 @@ describe Email::Receiver do
     expect { process(:old_destination) }.to raise_error(
       Email::Receiver::OldDestinationError
     )
+    expect(IncomingEmail.last.error).to eq("Email::Receiver::OldDestinationError")
 
     SiteSetting.disallow_reply_by_email_after_days = 0
     IncomingEmail.destroy_all