RFR: JDK-8269401: Merge "Exceptions" and "Errors" into "Exception Classes" [v2]

[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).

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8269401: Include Throwable in exception classes

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6061/files
  - new: https://git.openjdk.java.net/jdk/pull/6061/files/24626fd5..c1def9ec

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6061&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6061&range=00-01

  Stats: 15 lines in 4 files changed: 0 ins; 7 del; 8 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