[FLINK-6536] [cep] Improve error message in SharedBuffer::put().

flink-libraries/flink-cep/src/main/java/org/apache/flink/cep/nfa/SharedBuffer.java

@@ -97,10 +97,11 @@ public void put(
 
  // sanity check whether we've found the previous element
  if (previousSharedBufferEntry == null && previousValue != null) {
- throw new IllegalStateException("Could not find previous shared buffer entry with " +
+ throw new IllegalStateException("Could not find previous entry with " +
  "key: " + previousKey + ", value: " + previousValue + " and timestamp: " +
- previousTimestamp + ". This can indicate that the element belonging to the previous " +
- "relation has been already pruned, even though you expect it to be still there.");
+ previousTimestamp + ". This can indicate that either you did not implement " +
+ "the equals() and hashCode() methods of your input elements properly or that " +
+ "the element belonging to that entry has been already pruned.");
  }
 
  put(key, value, timestamp, previousSharedBufferEntry, version);