Remove exceptions support

[markandrus]

webidl2.js includes a function for parsing exceptions (I even went so far as to type these as IDLException in my typed branch); however, upon closer inspection, I don't believe WebIDL allows exceptions to be declared. See here:

An exception is a type of object that represents an error and which can be thrown or treated as a first class value by implementations. Web IDL does not allow exceptions to be defined, but instead has a number of pre-defined exceptions that specifications can reference and throw in their definition of operations, attributes, and so on.

I also don't see any reference to "exception" in the grammar, so I think this code should be removed (originally introduced here).

[marcoscaceres]

yeah, you used to be able to define exceptions many years ago. That's all dead code likely.