[minor] clean up message if no Customer from Quotation > Sales Order, fixes #3854
diff --git a/erpnext/selling/doctype/quotation/quotation.py b/erpnext/selling/doctype/quotation/quotation.py
index b5520ff..43dd675 100644
--- a/erpnext/selling/doctype/quotation/quotation.py
+++ b/erpnext/selling/doctype/quotation/quotation.py
@@ -155,6 +155,7 @@
 				else:
 					raise
 			except frappe.MandatoryError:
+				frappe.local.message_log = []
 				frappe.throw(_("Please create Customer from Lead {0}").format(lead_name))
 		else:
 			return customer_name