Prevent panic in ResponseError.Error()

[jhendrixMSFT]

If RawResponse is nil (unit test scenarios), print "Missing RawResponse" instead of panicing.

Fixes #21838

[RickWinter]

The change seems reasonable. Would guidance to users be to log and stop? We should give guidance on how to handle this case since now if it happened in production the customer needs to action

[jhendrixMSFT]

Customers should never be making programmatic choices based on the error message string as they are subject to change. Or did I misunderstand the question?

[RickWinter]

Customers should never be making programmatic choices based on the error message string as they are subject to change. Or did I misunderstand the question?

Before this change the app terminated with a panic. now it will return an error. The customer will need to do something... We should guide them on how to handle this newfound freedom.

[jhendrixMSFT]

SDKs only return an *azcore.ResponseError when the service returns a 400 (and possibly other) HTTP status code, which means we have a *http.Response. If a SDK method were ever to return an *azcore.ResponseError with a nil RawResponse field it means we have a bug either in an SDK or azcore.