Small improvement in maintenance scheduler

This improvement is intended to start disable blocking Maintenance Schedule submit, when the `Sales Person` does not have a `user_id` in the Employee!

The old logic try to use the `sales_person` email_id or the `owner` to create the event, but due some fails in the code, it dont occurs!
diff --git a/erpnext/support/doctype/maintenance_schedule/maintenance_schedule.py b/erpnext/support/doctype/maintenance_schedule/maintenance_schedule.py
index 5fab1ac..7d762f4 100644
--- a/erpnext/support/doctype/maintenance_schedule/maintenance_schedule.py
+++ b/erpnext/support/doctype/maintenance_schedule/maintenance_schedule.py
@@ -49,26 +49,28 @@
 
 			if d.sales_person not in email_map:
 				sp = frappe.get_doc("Sales Person", d.sales_person)
-				email_map[d.sales_person] = sp.get_email_id()
+				try:
+					email_map[d.sales_person] = sp.get_email_id()
+				except frappe.ValidationError:
+					pass
 
 			scheduled_date = frappe.db.sql("""select scheduled_date from
 				`tabMaintenance Schedule Detail` where sales_person=%s and item_code=%s and
 				parent=%s""", (d.sales_person, d.item_code, self.name), as_dict=1)
 
 			for key in scheduled_date:
-				if email_map[d.sales_person]:
-					description = "Reference: %s, Item Code: %s and Customer: %s" % \
-						(self.name, d.item_code, self.customer)
-					frappe.get_doc({
-						"doctype": "Event",
-						"owner": email_map[d.sales_person] or self.owner,
-						"subject": description,
-						"description": description,
-						"starts_on": cstr(key["scheduled_date"]) + " 10:00:00",
-						"event_type": "Private",
-						"ref_type": self.doctype,
-						"ref_name": self.name
-					}).insert(ignore_permissions=1)
+				description = "Reference: %s, Item Code: %s and Customer: %s" % \
+					(self.name, d.item_code, self.customer)
+				frappe.get_doc({
+					"doctype": "Event",
+					"owner": email_map.get(d.sales_person, self.owner),
+					"subject": description,
+					"description": description,
+					"starts_on": cstr(key["scheduled_date"]) + " 10:00:00",
+					"event_type": "Private",
+					"ref_type": self.doctype,
+					"ref_name": self.name
+				}).insert(ignore_permissions=1)
 
 		frappe.db.set(self, 'status', 'Submitted')