Missing check for error in client.go

[NautiluX]

Bug description

If this function call returns an error (I accidentally added a newline character to the end of the escalation policy id when called from GetEscalationPolicy), the call in the following line may result in a crash as req.Header may be nil.

However, if nil would be returned in case of an error to GetEscalationPolicy, this defer results in a crash as res.Body will be nil as well.

Expected behaviour

Erroneous input shouldn't result in a crash but the error being returned to the user.

[theckman]

This should have been fixed via #266, and should eventually be in v1.4.0+