nil check for server.

There is a bug in azure Ruby SDK which is not able to check the VM name
when the name is in upper case and returns a nil. This check will raise an
exception when the server object is nil.

lib/vagrant-azure/errors.rb

@@ -18,8 +18,8 @@ module VagrantPlugins
         error_key(:server_not_created)
       end
 
-      class CreateVMError < VagrantAzureError
-        error_key(:create_vm_error)
+      class CreateVMFailure < VagrantAzureError
+        error_key(:create_vm_failure)
       end
 
     end

locales/en.yml

@@ -32,6 +32,5 @@ en:
         to communicate to the remote machine. Please check the same and try again.
       server_not_created: |-
         Server not created. Error is: %{message}
-      create_vm_error: |-
-        Failed to create a VM in azure cloud with the following message.
-        %{message}
+      create_vm_failure: |-
+        There was some error in creating the VM.