Improve error message for invalid template or host keys on Self-Service API wizard

[LordHepipud]

The current message of the invalid template or host key input can be caused by the Icinga Director, as a host could have assigned a host key already with the Framework not knowing about the key

Expected Behavior

We should inform the user about this possibility

Current Behavior

We simply receive the message

Please enter either a template or your host key

which can result in various missunderstandings

Possible Solution

Add a hint for the user to look for

Steps to Reproduce (for bugs)

1. Register a host with a template
2. Remove the local key from the system
3. Try to register it again