Improve Net::SFTP::StatusException Exception Information

[sshaw]

Currently when a Net::SFTP::StatusException is raised we often get a #message like this:

open /some/path (4, "failure")

Not too informative.

When SFTP is run with logging at :verbose => :debug we see this helpful bit of infrormation:

 net.sftp.session[39ab0c]: SFTP User Exit Error: Bad Filename. SOME HELPFUL INFO"

How to get this information into the Net::SFTP::StatusException? I had a slightly-more-than-cursory look, and it does not look straightforward, but: looks can be deceiving.

[sshaw]

open /some/path (4, "failure")
Not too informative.

And status code 4 is "generic failure"