Ensure callback is called when envoking errorHanlder

A property, errorHanlder, was recently added which will be called
instead of throwing an error in the .flush() method. This is important
because errors in .flush() could, previously, only be handled via
process.on('uncaughtException', err => { ... }).

However, this property is currently unusable as, when the flush method
invokes this property, it fails to call the callbacks of the events
being flushed.

This commit makes sure the callbacks are called.

index.js

@@ -298,6 +298,7 @@ class Analytics {
       })
       .catch(err => {
         if (typeof this.errorHandler === 'function') {
+          done(err)
           return this.errorHandler(err)
         }
 

test.js

@@ -381,6 +381,23 @@ test('flush - do not throw on axios failure if errorHandler option is specified'
   t.true(errorHandler.calledOnce)
 })
 
+test('flush - evoke callback when errorHandler option is specified', async t => {
+  const errorHandler = spy()
+  const client = createClient({ errorHandler })
+  const callback = spy()
+
+  client.queue = [
+    {
+      message: 'error',
+      callback
+    }
+  ]
+
+  await t.notThrows(client.flush())
+  await delay(5)
+  t.true(callback.calledOnce)
+})
+
 test('flush - time out if configured', async t => {
   const client = createClient({ timeout: 500 })
   const callback = spy()