Added error handling to msfvenom when generating payloads

Added error handling to msfvenom when generating payloads

**Before**
    root@kali ~/metasploit-framework$ ./msfvenom -p windows/meterpreter/reverse_tcp                                                                                                          1 ↵ master
    /root/metasploit-framework/lib/msf/core/module.rb:620:in `validate': The following options failed to validate: LHOST. (Msf::OptionValidateError)
         from /root/metasploit-framework/lib/msf/core/encoded_payload.rb:65:in `generate'
         from /root/metasploit-framework/lib/msf/core/encoded_payload.rb:25:in `create'
         from /root/metasploit-framework/lib/msf/base/simple/payload.rb:53:in `generate_simple'
         from /root/metasploit-framework/lib/msf/base/simple/payload.rb:137:in `generate_simple'
         from ./msfvenom:343:in `<main>'
    root@kali ~/metasploit-framework$   


**After**
    root@kali ~/metasploit-framework$ ./msfvenom -p windows/meterpreter/reverse_tcp                                                                                                            master
    Error generating payload: The following options failed to validate: LHOST.
    root@kali ~/metasploit-framework$