QVT12 — clause 8.3.1.4 Exception needs to document the taxonomy of Exception types in QVT1.1

In particular, this taxonomy should explicitly include the AssertionFailed exception type that clause 8.2.2.20 refers to for an AssertExp.
Suggest defining a String message attribute for Exception; this would facilitate retrieving the message from a raise expression (clause 8.2.2.15)
Suggest defining AssertionFailed as a subtype of Exception.
Suggest defining 2 attributes in AssertionFailed corresponding to the severity and log expressions of the AssertExp (clause 8.2.2.20)

Yes. We need a toxonomy.
At the root is clearly Exception and this needs no parameters.
We need to introduce a derived StringException for the standard string-valued RaiseExp, and oops we
need to modify the grammar to support this shorthand.
The LogExp-valued AssertionFailed is another derivation of Exception; no severity since it is always
fatal.