Correct timer_entry error message on a non-timer

While

```puppet
systemd::manage_dropin{'custom.conf':
  ensure => present,
  unit   => 'my.service',
  timer_entry => {
   'Calendar' => 'Daily'
  },
}
```
was failing the error message gave incorrect information.

manifests/manage_dropin.pp

@@ -59,7 +59,7 @@
   Optional[Systemd::Unit::Path]    $path_entry              = undef,
 ) {
   if $timer_entry and $unit !~ Pattern['^[^/]+\.timer'] {
-    fail("Systemd::Manage_unit[${name}]: timer_entry is only valid for timer units")
+    fail("Systemd::Manage_dropin[${name}]: for unit ${unit} timer_entry is only valid for timer units")
   }
 
   if $path_entry and $unit !~ Pattern['^[^/]+\.path'] {