Show better error message when Model class isn't abstract

Describe the Bug
When you extend the ORM Model to your own Model and forget to set it to abstract, you get an Exception saying

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.lang.reflect.Field.getType()" because the return value of "org.javawebstack.orm.TableInfo.getField(String)" is null
	at org.javawebstack.orm.TableInfo.getIdType(TableInfo.java:282)
	at org.javawebstack.orm.TableInfo.analyzeTable(TableInfo.java:95)
	at org.javawebstack.orm.TableInfo.constructInfo(TableInfo.java:61)
	at org.javawebstack.orm.TableInfo.<init>(TableInfo.java:56)
	at org.javawebstack.orm.Repo.<init>(Repo.java:35)
	at org.javawebstack.orm.ORM.register(ORM.java:25)
	at org.javawebstack.orm.ORM.register(ORM.java:37)

To Reproduce
Steps to reproduce the behavior:

1. Forget to set your model class to abstract

Expected behavior
Maybe show some better error message which indicates that your base model isn't abstract yet.

Environment Information:

• Java Language Level 21
• ORM Version 1.0.3-SNAPSHOT

Riproduci il malfunzionamento registrando un modello non astratto, quindi parti da ORM.register e segui lo stack fino a Repo.java e TableInfo.java, in particolare constructInfo, analyzeTable e getIdType intorno alle righe segnalate. Controlla i percorsi esistenti di convalida dei modelli e gestione degli errori. Il lavoro è completato quando questo uso improprio produce un’eccezione chiara che identifichi che il modello di base deve essere astratto, invece della NullPointerException segnalata.