log errors into framework.log and show the callstack

See #4122 for discussion on this one.

Basicially this PR is all about logging a stacktracke when a module fails dua to coding errors (undefined method xxx on nil and so on).

Test module:
```
##
# This module requires Metasploit: http://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

require 'msf/core'

class Metasploit3 < Msf::Exploit::Remote
  Rank = ExcellentRanking

  def initialize(info = {})
    super(update_info(
    info,
    'Name'           => 'XXXX',
    'Description'    => %q{
      XXX
    },
    'Author'         =>
    [
      'Christian Mehlmauer' # metasploit module
    ],
    'License'        => MSF_LICENSE,
    'References'     =>
    [
      ['URL', 'https://www.google.com']
    ],
    'Privileged'     => false,
    'Platform'       => ['php'],
    'Arch'           => ARCH_PHP,
    'Targets'        => [['XXXX', {}]],
    'DefaultTarget'  => 0,
    'DisclosureDate' => 'Dec 3 2014'))
    end

    def exploit
      test = nil
      test.asdf
    end
  end
```

Example output of exploit_driver.rb:
```
msf exploit(test) > run

[*] Started reverse handler on 10.0.0.5:4444 
[-] Exploit failed: NoMethodError undefined method `asdf' for nil:NilClass
[-] Call stack:
[-]   /Users/firefart/.msf4/modules/exploits/test.rb:37:in `exploit'
[-]   /Users/firefart/Coding/metasploit-framework/lib/msf/core/exploit_driver.rb:205:in `job_run_proc'
[-]   /Users/firefart/Coding/metasploit-framework/lib/msf/core/exploit_driver.rb:166:in `run'
[-]   /Users/firefart/Coding/metasploit-framework/lib/msf/base/simple/exploit.rb:136:in `exploit_simple'
[-]   /Users/firefart/Coding/metasploit-framework/lib/msf/base/simple/exploit.rb:171:in `exploit_simple'
[-]   /Users/firefart/Coding/metasploit-framework/lib/msf/ui/console/command_dispatcher/exploit.rb:111:in `cmd_exploit'
[-]   /Users/firefart/Coding/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:427:in `run_command'
[-]   /Users/firefart/Coding/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:389:in `block in run_single'
[-]   /Users/firefart/Coding/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:383:in `each'
[-]   /Users/firefart/Coding/metasploit-framework/lib/rex/ui/text/dispatcher_shell.rb:383:in `run_single'
[-]   /Users/firefart/Coding/metasploit-framework/lib/rex/ui/text/shell.rb:200:in `run'
[-]   /Users/firefart/Coding/metasploit-framework/lib/metasploit/framework/command/console.rb:30:in `start'
[-]   /Users/firefart/Coding/metasploit-framework/lib/metasploit/framework/command/base.rb:82:in `start'
[-]   ./msfconsole:48:in `<main>'
```