fix(servicemanager): improve Secret-Data error handling

[MarlenKoch]

Problem:

The error message BindingCredentials can't be created from invalid secret data covers too many different scenarios.

crossplane-provider-btp/internal/clients/servicemanager/api_plan_resolver.go

Lines 45 to 47 in 84d2f41

	if binding.Clientid == nil || binding.Clientsecret == nil || binding.SmUrl == nil || binding.Url == nil || binding.Xsappname == nil {
	return BindingCredentials{}, errors.New(ErrInvalidSecretData)
	}

Solution:

This PR introduces more specific error messages that will be returned. It improves the user experience so that users can figure out the cause of the issue.

[cla-assistant]

All committers have signed the CLA.

[sdischer-sap]

LGTM! Good work!