Ability to retrieve errorcodes from commands sent via paramiko

Note that this means the output of execute() now has three values, not two. I have tried to fix all instances that might call execute().

I suppose it _might_ be possible to try and retrieve error code from the stdin passed to the function that is calling execute() itself, but it seems to me that we might as well get it straight from the source, rather than try to discern it after it's been passed along and possibly manipulated.

Also note that in order for this to show up in tests, the base calls will need to be modified, if desired, to pass the error code to the tests themselves.

In other words, existing def returns like

"return False if stderr else True"

...will not be enough to provide the error code. They'll need to be modified to return the error code value to the test.