Stdapi::AudioOutput.play_file: raise if file +path+ is not readable

[bcoles]

Rex::Post::Meterpreter::Extensions::Stdapi::AudioOutput.play_file(path) opens a channel to play a file path. The channel is opened immediately without any prior validation of path. Opening the channel is unnecessary if the file does not exist. Worse, failure to read the file will raise an error before channel.close is called, leaving the channel open.

This PR ensures that the file path is readable. No audio channel is opened if the file is not readable. It also ensures that the channel is closed in the event that something goes wrong.

Before

meterpreter > channel -l
No active channels.
meterpreter > play /asdf
[*] Playing /asdf...
[-] Error running command play: Errno::ENOENT No such file or directory @ rb_sysopen - /asdf
meterpreter > channel -l

    Id  Class  Type
    --  -----  ----
    11  3      audio_output

After

meterpreter > channel -l
No active channels.
meterpreter > play /asdf
[*] Playing /asdf...
[-] Error running command play: RuntimeError Could not read file: /asdf
meterpreter > channel -l
No active channels.