Throw an error when a packet can not be read to prevent infinite loop

[ashokadewit]

The method SSH2::_get_channel_packet() returns false when
the connection has been closed. This would cause an infinite loop
in SFTP::_get_sftp_packet because it keeps reading until the packet
buffer length is 4. To solve this a runtime exception is thrown when the
return value is false.

[ashokadewit]

This change is not needed in ^3.0 because there the method never returns false but throws an exception instead.

[terrafrost]

The 2.0 branch doesn't throw exceptions - it does user_error(). Exceptions are better, which is why 3.0 adopted them, but, in-so-far as 2.0 is concerned, they'd be a BC break, in-so-far a I'm concerned.

Here's what I did instead:

a02526e

Thanks!