No error handling on return code from posix_spawn

[evilrich]

Spoon performs no error handling on the return of _posixspawn. It also returns the process ID even when the spawn fails. For example:

> Spoon.spawn
 => 86511 

> Spoon.spawn('no_such_command')
 => 86529 

I have a fix for this here 9baec25d165bbb5568f378c62984c27a5d44bc6a

With this fix:

> Spoon.spawn('no_such_command')
Errno::ENOENT: No such file or directory - no_such_command
    from ./lib/spoon.rb:19:in `spawn'

[headius]

All good!