Check for non-empty 'Error' string in 'StartSessionResponse'

[buxomant]

Issue: When sending a StartSession request, we just check if there was an error reading the response message. However, we don't check if the response object itself contains an Error field.

Fix: Start checking for the Error field, and if present return an error accordingly.

[buxomant]

For context, the use case here was a device paired with a host, but pairing was broken after the device was factory reset. This was detected by idevicepair validate, but was not detected by go-ios, until this fix.