Sqllab error troubleshooting (#3680)

* provide troubleshooting link

* add option to append error troubleshooting link beneath sqllab error

superset/sql_lab.py

@@ -118,6 +118,10 @@ def execute_sql(
 
     def handle_error(msg):
         """Local method handling error while processing the SQL"""
+        troubleshooting_link = config["TROUBLESHOOTING_LINK"]
+        msg = "Error: {}. You can find common superset errors and their \
+            resolutions at: {}".format(msg, troubleshooting_link) \
+            if troubleshooting_link else msg
         query.error_message = msg
         query.status = QueryStatus.FAILED
         query.tmp_table_name = None