add exceptions

exceptions.md

-# Persistence:
+# Exceptions and Solutions
+
+## Persistence:
 
 ### Exception: java.lang.ClassCastException 
 * stack trace: class org.hibernate.mapping.SingleTableSubclass cannot be cast to class org.hibernate.mapping.RootClass
 * Possible Solution: Remove @Id Tags from Subclasses.
 
-### 
+### Exception: Unmapped class
+* Solution: Add class to persistence mapping in META-INF/persistence.xml
+
+### Exception: Primary key violation
+* Solution: an entity with this key already exists you idiot, change it!