Go to this link, pick your version of code running to reference the level of severity.

Generally you will see %PIX|ASA-X at the begining of loggin message where X is the severity level from 1 to 7

106100 severity 4

106001 severity 2

106002 severity 2

106006 severity 2

106007 severity 2

106010 severity 3

106012 severity 6

106013 severity 2

106014 severity 3

106015 severity 6

106016 severity 2

•Alert Messages, Severity 1

•Critical Messages, Severity 2

•Error Messages, Severity 3

•Warning Messages, Severity 4

•Notification Messages, Severity 5

•Informational Messages, Severity 6

•Debugging Messages, Severity 7

http://www.cisco.com/en/US/products/ps6120/products_system_message_guides_list.html

is there any show command on the firewall to show me that?

You have to reference the list of messages in above link, PIX/ASA will not explain the error message through cli.

Regards