Add categorization to the reporting of deserialization problems

[prmr]

Any kind of issue deserializing a JetUML file is currently reported as a DeserializationException, but there is a clear difference between:

• Syntactic errors (malformed Json)
• Structural errors (malformed diagram)
• Semantic errors (invalid diagram)

These should be reported as detected to facilitate integration with 3rd-party tools and libraries.