Errors returned from interactive emp run's are ugly

[ejholmes]

$ emp run bash -a does-not-exist-app
{"id":"not_found","message":"Request failed, the specified resource does not exist","url":""}

[ejholmes]

For the record, this happens because we don't check the Content-Type header here.

ClientConn.Do returns an *http.Response. If the Content-Type header is applciation/json then we should just do normal decoding. If it's application/vnd.empire.raw-stream, then we should do the io.Copy.

[ejholmes]

The other bug here is that, when this happens the command gets "stuck" and never returns. You can find the process and kill it (killall emp), but then your terminal is stuck because apparently the defer to reset the terminal never gets run. Then you have to $ reset the terminal.

/cc @bmarini

[ejholmes]

Fixed in #816