QENG-1257 scp_to Added IOError

Before, if the file doesn't exist, you would get a Result object back that had a nil exit_code.  

Talking with some people internally, we decided that if you get a Result object back, it implies that the command was run, which doesn't happen here.  

Now we raise an error, which is more indicative of an error in beaker setup, which is what's actually going on here.
Similar to above, these spec changes should be their own PR.