RFR: JDK-8269401: Merge "Exceptions" and "Errors" into "Exception Classes"

[Hannes Wallnöfer]

This is a conceptually simple change to merge the "Exception" and "Error" classes into a single "Exception Class" group.

The only change that is not strictly related to the merge is the removal of unused type subsets and related methods in `TypeElementCatalog.java` and the use of `Collections.emptySortedSet()` as the default return value in the `allClasses` method (I made sure it is never modified).

-------------

Commit messages:
 - JDK-8269401: Merge "Exceptions" and "Errors" into "Exception Classes"

Changes: https://git.openjdk.java.net/jdk/pull/6061/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6061&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269401
  Stats: 297 lines in 19 files changed: 16 ins; 217 del; 64 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6061.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6061/head:pull/6061

PR: https://git.openjdk.java.net/jdk/pull/6061