Merge "Exceptions" and "Errors" into "Exception Classes"

In line with the recent improvements in terminology, when listing different kinds of classes and interfaces, we should merge "Exceptions" and "Errors" into a single new tab, "Exception Classes".

From Alex:
 JLS ch.11 has always used that terminology, because differentiating an exception [object] from an exception class is important.

An exception is represented by an instance of the class Throwable (a direct subclass of Object) or one of its subclasses.
Throwable and all its subclasses are, collectively, the exception classes.
The unchecked exception classes are the run-time exception classes and the error classes.
The checked exception classes are all exception classes other than the unchecked exception classes.